update gcc call in Makefile

Compilation through `make` fails with the following error message on my laptop.

> /usr/bin/ld: /tmp/cclmBbxI.o: undefined reference to symbol 'PEM_read_PrivateKey@@OPENSSL_0.9.8'
> /usr/bin/ld: note: 'PEM_read_PrivateKey@@OPENSSL_0.9.8' is defined in DSO /usr/lib/libcrypto.so.0.9.8 so try adding it to the linker command line
> /usr/lib/libcrypto.so.0.9.8: could not read symbols: Invalid operation

Linking the crypto library (as advised in the error message) fixes the problem.