python: only build required files before asking setup to build

We only need our config.h and the swig wrapper, so skip building the
entire library only to rebuild it again via the amalgamation.

setup.py

@@ -40,18 +40,17 @@
     # Run the autotools/make build up front to generate our sources,
     # then build using the standard Python ext module machinery.
     # (Windows requires source generation to be done separately).
-    import multiprocessing
     import subprocess
 
     abs_path = os.path.dirname(os.path.abspath(__file__)) + '/'
 
-    def call(args):
-        subprocess.check_call(args, cwd=abs_path, env=configure_env)
+    def call(args, cwd=abs_path):
+        subprocess.check_call(args, cwd=cwd, env=configure_env)
 
     call(['./tools/cleanup.sh'])
     call(['./tools/autogen.sh'])
     call(['./configure'] + CONFIGURE_ARGS)
-    call(['make', '-j{}'.format(multiprocessing.cpu_count())])
+    call(['make', 'swig_python/swig_python_wrap.c'], abs_path + 'src/')
 
 define_macros=[
     ('SWIG_PYTHON_BUILD', None),
@@ -66,7 +65,11 @@ def call(args):
     './src/secp256k1/include',
     ]
 
-extra_compile_args = ['-flax-vector-conversions']
+if is_windows:
+    include_dirs = ['./src/amalgamation/windows_config'] + include_dirs
+    extra_compile_args = []
+else:
+    extra_compile_args = ['-flax-vector-conversions']
 
 wally_ext = Extension(
     '_wallycore',