setup.py: support optionally building libwally-core as a shared library

Iff the environment contains WALLY_BUILD_SHARED=1, then:

 * setup.py will configure and build libwally-core as a shared library;
 * the Python native extension library will not contain any
   libwally-core or libsecp256k1 code; and
 * the dynamic linker will need to find and load libwallycore.so.1
   whenever the Python native extension is loaded.

setup.py

@@ -3,9 +3,16 @@
 import copy, os, platform, shutil
 import distutils.sysconfig
 
-CONFIGURE_ARGS = ['--disable-shared', '--enable-static', '--with-pic',
+build_shared = os.environ.get('WALLY_BUILD_SHARED', '').lower() not in ['', '0', 'false', 'no', 'n', 'off']
+if build_shared:
+    CONFIGURE_ARGS = ['--enable-shared', '--disable-static']
+else:
+    CONFIGURE_ARGS = ['--disable-shared', '--enable-static', '--with-pic']
+
+CONFIGURE_ARGS += [
     '--enable-swig-python', '--enable-python-manylinux',
-    '--disable-swig-java', '--disable-tests', '--disable-dependency-tracking']
+    '--disable-swig-java', '--disable-tests', '--disable-dependency-tracking',
+    ]
 
 distutils_env = distutils.sysconfig.get_config_vars()
 configure_env = copy.deepcopy(os.environ)
@@ -82,8 +89,8 @@ def call(args):
     '_wallycore',
     define_macros=define_macros,
     include_dirs=include_dirs,
-    library_dirs=['src/.libs', 'src/secp256k1/.libs'],
-    libraries=['wallycore', 'secp256k1'],
+    library_dirs=['src/.libs'] + ([] if build_shared else ['src/secp256k1/.libs']),
+    libraries=['wallycore'] + ([] if build_shared else ['secp256k1']),
     extra_compile_args=extra_compile_args,
     sources=[
         'src/swig_python/swig_wrap.c' if is_windows else 'src/swig_python/swig_python_wrap.c',