Enable building our internal LLVM with modules.

interpreter/llvm/Module.mk

@@ -175,6 +175,9 @@ $(LLVMDEPO): $(LLVMDEPS)
 		if [ $(GCCTOOLCHAIN) ]; then \
 			LLVM_CFLAGS="$(LLVM_CFLAGS) --gcc-toolchain=$(GCCTOOLCHAIN) "; \
 		fi; \
+		if [ $(CXXMODULES) ]; then \
+			LLVM_CXXMODULES=" -DLLVM_ENABLE_MODULES=ON "; \
+		fi; \
 		echo "*** Configuring LLVM in $(dir $@) ..."; \
 		mkdir -p $(dir $@) && \
 		cd $(dir $@)  && \
@@ -205,6 +208,7 @@ $(LLVMDEPO): $(LLVMDEPS)
 		$$LLVM_32BITS \
 		-DLLVM_TARGETS_TO_BUILD=host \
 		$$LLVM_GCC_TOOLCHAIN \
+		$$LLVM_CXXMODULES \
 		-DCMAKE_C_COMPILER=$$LLVMCC \
                 -DCMAKE_CXX_COMPILER=$$LLVMCXX \
 		-DCMAKE_CXX_FLAGS="$$LLVM_CFLAGS" \