configure.ac: support --with-system-secp256k1

Libwally-core, in its stock configuration, compiles and statically links
against a private copy of libsecp256k1_zkp. However, Gentoo unbundles
libsecp256k1_zkp and instead links libwally-core against a system-wide
shared libsecp256k1_zkp with headers in /usr/include/secp256k1_zkp and a
libsecp256k1_zkp.pc that specifies the relevant CFLAGS and LIBS.

Implement a --with-system-secp256k1 configure option to allow compiling
and linking against a system-installed libsecp256k1. Pass the user-
specified package name (or 'libsecp256k1' or 'libsecp256k1_zkp' by
default, depending on --enable-standard-secp) to PKG_CHECK_MODULES to
find the CFLAGS and LIBS of a system-installed libsecp256k1. Call
AC_CHECK_FUNCS to assert that the required modules are present in the
system-installed libsecp256k1.

If the user does not specify --with-system-secp256k1 (or specifies
--without-system-secp256k1), then the pre-existing behavior is
preserved: namely, the build will use the bundled copy of
libsecp256k1_zkp.

setup.py is tweaked to detect if the libwally-core library has already
been built and, in that case, elide rebuilding it. This supports a use
case of linking libwallycore.so against a system-installed libsecp256k1
and then linking the Python extension against that libwallycore.so. In
such a scenario, the Python native extension library contains no code
from libwally-core or libsecp256k1 but instead simply specifies a
dynamic link with libwallycore.so.1, and libwallycore.so.1 contains no
code from libsecp256k1 but instead simply specifies a dynamic link with
libsecp256k1_zkp.so.0. This is the "full dynamic link" holy grail. Note
that the default case is nearly the opposite: all libsecp256k1 and
libwally-core code is embedded into the Python native extension library.

configure.ac

@@ -259,8 +259,42 @@ fi
 #
 # libsecp256k1
 #
-libsecp256k1_CFLAGS='-I$(top_srcdir)/src/secp256k1/include'
-libsecp256k1_LIBS='$(top_srcdir)/src/secp256k1/libsecp256k1.la'
+AC_ARG_WITH([system-secp256k1],
+    [AS_HELP_STRING([[--with-system-secp256k1[=PKG]]],
+        [build using system-installed libsecp256k1 instead of bundled, passing PKG (default: libsecp256k1 or libsecp256k1_zkp, depending on --enable-standard-secp) to pkg-config (default: no)])],
+    [AS_IF([test "x$withval" = xyes],
+        [AM_COND_IF([BUILD_STANDARD_SECP], [with_system_secp256k1=libsecp256k1], [with_system_secp256k1=libsecp256k1_zkp])])],
+    [with_system_secp256k1=no])
+AM_CONDITIONAL([LINK_SYSTEM_SECP256K1], [test "x$with_system_secp256k1" != xno])
+AM_COND_IF([LINK_SYSTEM_SECP256K1], [
+    saved_LIBS=$LIBS
+    PKG_CHECK_MODULES([libsecp256k1], [$with_system_secp256k1])
+    LIBS="$libsecp256k1_LIBS $LIBS"
+    missing_modules=
+    AC_DEFUN([CHECK_MODULE], [
+        AC_CHECK_FUNCS([$2], [], [missing_modules="${missing_modules} $1"])
+    ])
+    CHECK_MODULE([ecdh], [secp256k1_ecdh])
+    CHECK_MODULE([extrakeys], [secp256k1_xonly_pubkey_parse])
+    CHECK_MODULE([recovery], [secp256k1_ecdsa_recover])
+    CHECK_MODULE([schnorrsig], [secp256k1_schnorrsig_verify])
+    AM_COND_IF([BUILD_STANDARD_SECP], [], [
+        CHECK_MODULE([ecdsa-s2c], [secp256k1_ecdsa_s2c_sign])
+    ])
+    AM_COND_IF([BUILD_ELEMENTS], [
+        CHECK_MODULE([generator], [secp256k1_generator_parse])
+        CHECK_MODULE([rangeproof], [secp256k1_rangeproof_verify])
+        CHECK_MODULE([surjectionproof], [secp256k1_surjectionproof_initialize])
+        CHECK_MODULE([whitelist], [secp256k1_whitelist_sign])
+    ])
+    AS_IF([test -n "${missing_modules}"], [
+        AC_MSG_ERROR([system-installed $with_system_secp256k1 does not support these required modules:${missing_modules}])
+    ])
+    LIBS=$saved_LIBS
+], [
+    libsecp256k1_CFLAGS='-I$(top_srcdir)/src/secp256k1/include'
+    libsecp256k1_LIBS='$(top_srcdir)/src/secp256k1/libsecp256k1.la'
+])
 AC_SUBST([libsecp256k1_CFLAGS])
 AC_SUBST([libsecp256k1_LIBS])
 
@@ -395,6 +429,8 @@ export AR_FLAGS
 export LD
 export LDFLAGS
 
-AX_SUBDIRS_CONFIGURE([src/secp256k1], [[--disable-shared], [--enable-static], [--with-pic], [--enable-experimental], [--enable-module-ecdh], [--enable-module-recovery], [--enable-module-ecdsa-s2c], [--enable-module-rangeproof], [--enable-module-surjectionproof], [--enable-module-whitelist], [--enable-module-generator], [--enable-module-extrakeys], [--enable-module-schnorrsig], [$secp256k1_test_opt], [--enable-exhaustive-tests=no], [--enable-benchmark=no], [--disable-dependency-tracking], [$secp_asm]])
+AM_COND_IF([LINK_SYSTEM_SECP256K1], [], [
+	AX_SUBDIRS_CONFIGURE([src/secp256k1], [[--disable-shared], [--enable-static], [--with-pic], [--enable-experimental], [--enable-module-ecdh], [--enable-module-recovery], [--enable-module-ecdsa-s2c], [--enable-module-rangeproof], [--enable-module-surjectionproof], [--enable-module-whitelist], [--enable-module-generator], [--enable-module-extrakeys], [--enable-module-schnorrsig], [$secp256k1_test_opt], [--enable-exhaustive-tests=no], [--enable-benchmark=no], [--disable-dependency-tracking], [$secp_asm]])
+])
 
 AC_OUTPUT

setup.py

@@ -51,13 +51,15 @@
 
     abs_path = os.path.dirname(os.path.abspath(__file__)) + '/'
 
-    def call(args):
-        subprocess.check_call(args, cwd=abs_path, env=configure_env)
+    if not os.path.isfile(abs_path + 'src/.libs/libwallycore' + ('.so' if build_shared else '.a')):
 
-    call(['./tools/cleanup.sh'])
-    call(['./tools/autogen.sh'])
-    call(['./configure'] + CONFIGURE_ARGS)
-    call(['make', '-j{}'.format(multiprocessing.cpu_count())])
+        def call(args):
+            subprocess.check_call(args, cwd=abs_path, env=configure_env)
+
+        call(['./tools/cleanup.sh'])
+        call(['./tools/autogen.sh'])
+        call(['./configure'] + CONFIGURE_ARGS)
+        call(['make', '-j{}'.format(multiprocessing.cpu_count())])
 
 define_macros=[
     ('SWIG_PYTHON_BUILD', None),

src/Makefile.am

@@ -208,7 +208,9 @@ endif # SHARED_BUILD_ENABLED
 libwallycore_la_CFLAGS = -I$(top_srcdir) -I$(srcdir)/ccan $(libsecp256k1_CFLAGS) -DWALLY_CORE_BUILD=1 $(AM_CFLAGS)
 libwallycore_la_LIBADD = $(libsecp256k1_LIBS) $(noinst_LTLIBRARIES)
 
+if !LINK_SYSTEM_SECP256K1
 SUBDIRS = secp256k1
+endif
 
 TESTS =
 noinst_PROGRAMS =