Adding support to LLVM Clang and creating configure flags for gcov and gprof

* Added some checks to detect Clang compiler to add the correct flags
* Added a check to change the default '-g -O2' flags on GCC by '-g -O0 -Wall' if gcov or gprof switches are activated
* Added gcov and gprof flags on  configure script

WARNING: It's my first modification on GNU Autotools build system, so it's very probably not the best solution. 

Maybe you could add clang in .travis.yml to test it.

Thanks for your work