Give control over gmp support

--- pycrypto/setup.py
+++ pycrypto/setup.py
@@ -116,10 +116,8 @@
         build_ext.build_extensions(self)
 
     def detect_modules (self):
-        lib_dirs = self.compiler.library_dirs + ['/lib', '/usr/lib']
-        inc_dirs = self.compiler.include_dirs + ['/usr/include']
         exts = []
-        if (self.compiler.find_library_file(lib_dirs, 'gmp')):
+        if os.environ.has_key("USE_GMP") and os.environ["USE_GMP"] == "1":
             exts.append(Extension("Crypto.PublicKey._fastmath",
                                   include_dirs=['src/'],
                                   libraries=['gmp'],