Fix build with clang

There are two instances when building the tests where OpenBLAS fails to
build with OpenMP and clang due to library paths getting reset as flags
are set rather than appended. This seems to only affect certain
clang/libomp installations, but if it's already grabbing the correct
library paths we might as well use them.

ctest/Makefile

@@ -208,7 +208,7 @@ FLDFLAGS = $(FFLAGS:-fPIC=) $(LDFLAGS)
 ifeq ($(USE_OPENMP), 1)
 ifeq ($(F_COMPILER), GFORTRAN)
 ifeq ($(C_COMPILER), CLANG)
-CEXTRALIB = -lomp
+CEXTRALIB += -lomp
 endif
 endif
 ifeq ($(F_COMPILER), NAG)

test/Makefile

@@ -265,7 +265,7 @@ FLDFLAGS = $(FFLAGS:-fPIC=) $(LDFLAGS)
 ifeq ($(USE_OPENMP), 1)
 ifeq ($(F_COMPILER), GFORTRAN)
 ifeq ($(C_COMPILER), CLANG)
-CEXTRALIB = -lomp
+CEXTRALIB += -lomp
 endif
 endif
 ifeq ($(F_COMPILER), NAG)