Revert "Remove more restrictions when building with modules."

This reverts commit cb662602.

rootcling version is tied to our internal version of clang. Passing in -fmodules
would use the modules already built from the outside compiler, whose version
might differ. The module files are not compatible among the clang versions.

CMakeLists.txt

@@ -149,6 +149,11 @@ get_property(__clingetcpch GLOBAL PROPERTY CLINGETCPCH)
 
 
 string(REGEX REPLACE "[ ]-" ";-" CMAKE_CXX_FLAGS_SEPARATE ${CMAKE_CXX_FLAGS})
+if(cxxmodules)
+  # rootcling doesn't know what to do with these flags.
+  # FIXME: Disable until until somebody teaches it.
+  string(REPLACE "${ROOT_CXXMODULES_FLAGS}" "" CMAKE_CXX_FLAGS_SEPARATE ${CMAKE_CXX_FLAGS_SEPARATE})
+endif(cxxmodules)
 add_custom_command(OUTPUT etc/dictpch/allLinkDefs.h
                           etc/dictpch/allHeaders.h
                           etc/dictpch/allCppflags.txt

Makefile

@@ -1137,6 +1137,11 @@ changelog:
 releasenotes:
 	@$(MAKERELNOTES)
 ROOTCLING_CXXFLAGS := $(CXXFLAGS)
+# rootcling doesn't know what to do with these flags.
+# FIXME: Disable until until somebody teaches it.
+ifeq ($(CXXMODULES),yes)
+ROOTCLING_CXXFLAGS := $(filter-out $(ROOT_CXXMODULES_CXXFLAGS),$(CXXFLAGS))
+endif
 
 $(ROOTPCH): $(MAKEPCH) $(ROOTCLINGSTAGE1DEP) $(ALLHDRS) $(CLINGETCPCH) $(ORDER_) $(ALLLIBS)
 	@$(MAKEPCHINPUT) $(ROOT_SRCDIR) "$(MODULES)" $(CLINGETCPCH) -- $(ROOTCLING_CXXFLAGS)