Resolve issue #57 (#58)

* Check if libsecp256k1 has ecdh modules

* Download libsecp256k1 when necessary

When building the clib, it is possible that libsecp256k1's sources are
not present causing the build to fail. This ensures they are downloaded
before starting the build.

setup.py

@@ -139,6 +139,10 @@ class build_clib(_build_clib):
             if e.errno != errno.EEXIST:
                 raise
 
+        if not os.path.exists(absolute('libsecp256k1')):
+            # library needs to be downloaded
+            self.get_source_files()
+
         if not os.path.exists(absolute('libsecp256k1/configure')):
             # configure script hasn't been generated yet
             autogen = absolute('libsecp256k1/autogen.sh')

setup_support.py

@@ -69,6 +69,11 @@ def _find_lib():
     ffi = FFI()
     try:
         ffi.dlopen("secp256k1")
+        if os.path.exists('/usr/include/secp256k1_ecdh.h'):
+            return True
+        else:
+            # The system library lacks the ecdh module
+            return False
     except OSError:
         if 'LIB_DIR' in os.environ:
             for path in glob.glob(os.path.join(os.environ['LIB_DIR'], "*secp256k1*")):