setup.py: support pkgconfig to use system-installed libsecp256k1_zkp

If the environment contains WALLY_ABI_PY_WHEEL_USE_PKG_SECP256K1=1 (and
the amalgamated objects are not being built), then the pkgconfig Python
module will be called to find the include path for the system-installed
libsecp256k1_zkp, and the secp256k1 submodule will not be checked out.

WALLY_ABI_PY_WHEEL_USE_PKG_SECP256K1 can alternatively be set to a
specific package name if the pkg-config name of the system-installed
libsecp256k1-zkp is something other than 'libsecp256k1_zkp'.

setup.py

@@ -26,11 +26,44 @@
         '\nuse it. Set WALLY_ABI_PY_WHEEL_USE_LIB=static to link with the static library.\n',
         file=sys.stderr)
 
+def str_to_bool(str, if_none=ValueError, if_invalid=ValueError):
+    if str is None:
+        default = if_none
+    else:
+        lstr = str.lower()
+        if lstr in ('0', 'n', 'no', 'f', 'false', 'off'):
+            return False
+        elif lstr in ('1', 'y', 'yes', 't', 'true', 'on'):
+            return True
+        default = if_invalid
+    if isinstance(default, BaseException):
+        raise default('Invalid flag value: {}'.format(str))
+    return default
+
+def pkg_default(pkg, if_true, if_false=None):
+    b = str_to_bool(pkg, None, None)
+    return if_true if b == True else if_false if b == False else pkg
+
+PKG_SECP256K1 = pkg_default(
+    os.environ.get('WALLY_ABI_PY_WHEEL_USE_PKG_SECP256K1'), 'libsecp256k1_zkp')
+
+if PKG_SECP256K1 is not None:
+    try:
+        import pkgconfig
+    except ImportError:
+        print('Error: You specified WALLY_ABI_PY_WHEEL_USE_PKG_SECP256K1, but you do not have'
+            '\nthe pkgconfig Python module installed.',
+            file=sys.stderr)
+        sys.exit(127)
+
 def call(args, cwd=ABS_PATH):
     subprocess.check_call(args, cwd=cwd, env=CONFIGURE_ENV)
 
-if not os.path.exists('src/secp256k1/Makefile.am'):
-    # Sync libsecp-zkp
+if (USE_LIB == 'no' or PKG_SECP256K1 is None) and not os.path.exists(
+        'src/.libs/libwallycore.so' if USE_LIB == 'shared' else
+        'src/secp256k1/.libs/libsecp256k1.a' if USE_LIB == 'static' else
+        'src/secp256k1/Makefile.am'):
+    # Sync libsecp256k1-zkp (only if needed)
     call(['git','submodule','init'])
     call(['git','submodule','sync','--recursive'])
     call(['git','submodule','update','--init','--recursive'])
@@ -90,7 +123,6 @@ def call(args, cwd=ABS_PATH):
 include_dirs=[
     './',
     './src',
-    './src/secp256k1/include',
     ]
 library_dirs = [
     ]
@@ -103,13 +135,18 @@ def call(args, cwd=ABS_PATH):
 if USE_LIB == 'no':
     include_dirs += [
         './src/ccan',
+        './src/secp256k1/include',
         ]
     sources += [
         'src/amalgamation/combined.c',
         'src/amalgamation/combined_ccan.c',
         'src/amalgamation/combined_ccan2.c',
         ]
 else:
+    if PKG_SECP256K1 is None:
+        include_dirs += ['./src/secp256k1/include']
+    else:
+        include_dirs += pkgconfig.parse(PKG_SECP256K1)['include_dirs']
     library_dirs += ['src/.libs']
     libraries += ['wallycore']
     if USE_LIB == 'static':