Use the evironment for the CC binary

See discussion at https://groups.google.com/forum/#!topic/ossec-list/FOTncDNnNk0 

The ossec-lua addition included a regression on @cgzones changes for using clang correctly.  This corrects that regression (as suggest by cgzones on the mailing list). 

I think this should also be merged into stable for the 2.8 release as the ossec-lua introduced a regression into clang builds.  

Please note travis will not pick up this try of errors due to gcc still being installed.