[omp] in ggttmad fix OMP link flags for clang (madgraph5#578)

I added -lomp because "ldd check.exe" showed that libomp was used

epochX/cudacpp/gg_tt.mad/SubProcesses/cudacpp.mk

@@ -171,8 +171,6 @@ endif
 # Set the default OMPFLAGS choice
 ifneq ($(shell $(CXX) --version | egrep '^Intel'),)
 override OMPFLAGS = # disable OpenMP MT on Intel (ok without nvcc, not ok with nvcc)
-else ifneq ($(shell $(CXX) --version | egrep '^(clang|Apple clang)'),)
-override OMPFLAGS = # disable OpenMP MT on clang (not ok without or with nvcc)
 else
 override OMPFLAGS = -fopenmp
 ###override OMPFLAGS = # disable OpenMP MT (default before #575)
@@ -613,8 +611,8 @@ endif
 ifneq ($(OMPFLAGS),)
 ifneq ($(shell $(CXX) --version | egrep '^Intel'),)
 ###$(testmain): LIBFLAGS += -qopenmp -static-intel # see https://stackoverflow.com/questions/45909648/explicitly-link-intel-icpc-openmp
-else ifneq ($(shell $(CXX) --version | egrep '^(clang|Apple clang)'),)
-###$(testmain): LIBFLAGS += ??? # OpenMP on clang is not yet supported in cudacpp...
+else ifneq ($(shell $(CXX) --version | egrep '^(clang|Apple clang)'),) # see #578
+$(testmain): LIBFLAGS += -lomp
 else
 $(testmain): LIBFLAGS += -lgomp
 endif

epochX/cudacpp/gg_tt.mad/SubProcesses/makefile

@@ -97,6 +97,9 @@ endif
 ifneq ($(shell $(CXX) --version | egrep '^Intel'),)
 override OMPFLAGS = -fopenmp
 LINKLIBS += -lintlc # undefined reference to `_intel_fast_memcpy'
+else ifneq ($(shell $(CXX) --version | egrep '^(clang|Apple clang)'),) # see #578
+override OMPFLAGS = -fopenmp
+LINKLIBS += -lomp
 else
 override OMPFLAGS = -fopenmp
 endif