setup.py: fully automate secp256k1 extension module build

The secp256k1 library is now cloned from Github and built automatically.  It’s
linked statically to the extension mod, so that the resulting wheel can run on
a system without libsecp256k1 installed.

setup.py

@@ -1,16 +1,39 @@
 #!/usr/bin/env python3
 
+import sys,os
 from setuptools import setup,Extension
+from setuptools.command.build_ext import build_ext
 from subprocess import run,PIPE
 
+cache_path = os.path.join(os.environ['HOME'],'.cache','mmgen')
+ext_path = os.path.join(cache_path,'secp256k1')
+
+def build_libsecp256k1():
+	if not os.path.exists(cache_path):
+		os.makedirs(cache_path)
+	if not os.path.exists(ext_path):
+		run(['git','clone','https://github.com/bitcoin-core/secp256k1.git'],check=True,cwd=cache_path)
+	if not os.path.exists(os.path.join(ext_path,'.libs/libsecp256k1.a')):
+		run(['./autogen.sh'],check=True,cwd=ext_path)
+		run(['./configure','CFLAGS=-g -O2 -fPIC'],check=True,cwd=ext_path)
+		run(['make','-j4'],check=True,cwd=ext_path)
+
 have_msys2 = run(['uname','-s'],stdout=PIPE,check=True).stdout.startswith(b'MSYS_NT')
 if have_msys2:
 	print('MSYS2 system detected')
 
-setup(ext_modules=[Extension(
-	name         = 'mmgen.secp256k1',
-	sources      = ['extmod/secp256k1mod.c'],
-	libraries    = ['secp256k1'] + ([],['gmp'])[have_msys2],
-	library_dirs = ['/usr/local/lib',r'C:\msys64\mingw64\lib',r'C:\msys64\usr\lib'],
-	include_dirs = ['/usr/local/include',r'C:\msys64\mingw64\include',r'C:\msys64\usr\include'],
-	)])
+class my_build_ext(build_ext):
+	def build_extension(self,ext):
+		build_libsecp256k1()
+		build_ext.build_extension(self,ext)
+
+setup(
+	cmdclass = { 'build_ext': my_build_ext },
+	ext_modules = [Extension(
+		name         = 'mmgen.secp256k1',
+		sources      = ['extmod/secp256k1mod.c'],
+		libraries    = ([],['gmp'])[have_msys2],
+		extra_objects = [os.path.join(ext_path,'.libs/libsecp256k1.a')],
+		include_dirs = [os.path.join(ext_path,'include')],
+	)]
+)