build: Touch llvm-config after building

llvm-config is the file we base our LLVM build rule on, but rebuilding LLVM
doesn't always rebuild it, so touch.

Author: brson

mk/llvm.mk

@@ -14,7 +14,7 @@ ifeq ($(CFG_LLVM_ROOT),)
 $$(LLVM_CONFIG_$(1)): $$(LLVM_DEPS)
 	@$$(call E, make: llvm)
 	$$(Q)$$(MAKE) -C $$(CFG_LLVM_BUILD_DIR_$(1)) $$(CFG_LLVM_BUILD_ENV)
-
+	$$(Q)touch $$(LLVM_CONFIG_$(1))
 endif
 
 endef