Do not force gcc options on non-gcc compilers

fixes compile failure with pgi 18.10 as reported on OpenBLAS-users

Author: martin-frbg

Makefile.x86_64

@@ -28,11 +28,15 @@ endif
 ifeq ($(CORE), HASWELL)
 ifndef DYNAMIC_ARCH
 ifndef NO_AVX2
+ifeq ($(C_COMPILER), GCC)
 CCOMMON_OPT += -mavx2
+endif
+ifeq $(F_COMPILER), GFORTRAN)
 FCOMMON_OPT += -mavx2
 endif
 endif
 endif
+endif