Allow override system libsecp with env. variable (#35)

* allow override system libsecp with env. variable

* rename secp override variable

* Update setup_support.py

setup_support.py

@@ -68,6 +68,9 @@ def build_flags(library, type_, path):
 
 
 def _find_lib():
+    if 'COINCURVE_IGNORE_SYSTEM_LIB' in os.environ:
+        return False
+
     from cffi import FFI
     ffi = FFI()
     try: