Build process update: make clang link against our new libstdc++

Previously it would always link against the system libstdc++; not sure how
that was working.

Also, put that in its own included makefile so that we can track the dependence
on the configuration parameters (otherwise you would have to run a configuration
manually to get this change).

src/Makefile

@@ -454,11 +454,21 @@ llvm_configure:
 	rm -f $(LLVM_BUILD)/Makefile.config
 	$(MAKE) $(LLVM_CONFIGURATION)
 
-LLVM_CONFIGURE_LINE := CXX=$(GPP) $(LLVM_SRC)/configure --enable-targets=host --with-gcc-toolchain=$(GCC_DIR)
+# Put the llvm_configure line in its own file, so that we can force an llvm reconfigure
+# if we change the configuration parameters.
+# (All non-llvm build targets get rebuilt if the main Makefile is touched, but that is too
+# expensive to do for the llvm ones.)
+LLVM_CONFIG_INCL := Makefile.llvmconfig
+# Sets LLVM_CONFIGURE_LINE:
+include $(LLVM_CONFIG_INCL)
+ifeq (,$(LLVM_CONFIGURE_LINE))
+$(error "did not set configure line")
+endif
 ifneq ($(ENABLE_INTEL_JIT_EVENTS),0)
 LLVM_CONFIGURE_LINE += --with-intel-jitevents
 endif
-$(LLVM_CONFIGURATION): $(LLVM_SRC)/configure | $(LLVM_SRC)/_patched
+
+$(LLVM_CONFIGURATION): $(LLVM_SRC)/configure $(LLVM_CONFIG_INCL) | $(LLVM_SRC)/_patched
 	mkdir -p $(LLVM_BUILD)
 	cd $(LLVM_BUILD) ; \
 	$(LLVM_CONFIGURE_LINE)

src/Makefile.llvmconfig

+# included by Makefile
+
+LLVM_CONFIGURE_LINE := CXX=$(GPP) LDFLAGS="-Wl,-rpath,$(GCC_DIR)/lib64" $(LLVM_SRC)/configure --enable-targets=host --with-gcc-toolchain=$(GCC_DIR)