(fea) selectable download using pkg-config

setup.py

@@ -78,22 +78,25 @@ def download_library(command):
 class egg_info(_egg_info):
     def run(self):
         # Ensure library has been downloaded (sdist might have been skipped)
-        download_library(self)
+        if not has_system_lib():
+            download_library(self)
 
         _egg_info.run(self)
 
 
 class sdist(_sdist):
     def run(self):
-        download_library(self)
+        if not has_system_lib():
+            download_library(self)
         _sdist.run(self)
 
 
 if _bdist_wheel:
 
     class bdist_wheel(_bdist_wheel):
         def run(self):
-            download_library(self)
+            if not has_system_lib():
+                download_library(self)
             _bdist_wheel.run(self)
 
 
@@ -113,7 +116,8 @@ def finalize_options(self):
 
     def get_source_files(self):
         # Ensure library has been downloaded (sdist might have been skipped)
-        download_library(self)
+        if not has_system_lib():
+            download_library(self)
 
         return [
             absolute(os.path.join(root, filename))

setup_support.py

@@ -64,7 +64,9 @@ def _find_lib():
     ffi = FFI()
     try:
         ffi.dlopen('secp256k1')
-        return os.path.exists('/usr/include/secp256k1_ecdh.h')
+        include_path = subprocess.check_output(['pkg-config', '--static', '--cflags-only-I', 'libsecp256k1'])  # noqa S603
+        include_path = include_path.decode('UTF-8').strip().split()[0][2:]
+        return os.path.exists(f'{include_path}/secp256k1_ecdh.h')
     except OSError:
         if 'LIB_DIR' in os.environ:
             for path in glob.glob(os.path.join(os.environ['LIB_DIR'], '*secp256k1*')):