Handle the case for no exe extension for Cygwin

On, at least,  Cygwin perl-5.14.2-3, ```$Config{cc}``` is ```'gcc-4'``` and ```$Config{_exe}``` is ```'.exe'```. On the other hand, there is no ```gcc-4.exe```. ```/usr/bin/gcc-4``` is a symlink to ```/usr/bin/gcc.exe```. So, most tests are skipped by ```Couldn't find your C compiler``` like http://www.cpantesters.org/cpan/report/4090954b-71a6-1014-9a36-9dd55cf4dae8

To fix it, I added check for the case of no extension even if ```$Config{_exe}``` is defined.