Fix gcc detection

I am not a bash expert, but with the current grep line in the configure script, I get the following result when trying to configure mlt (using Ubuntu 14.10):

gcc: error: (Ubuntu: No such file or directory
gcc: error: 4.9.1-15ubuntu1): No such file or directory
gcc: error: 4.9.1: No such file or directory
gcc: fatal error: no input files
compilation terminated.

My patch fixes the detection of GCC on my system