More reliable and consistent method of cancelling -Werror*

Adding -Wno-error is more reliable and simple than trying to modify existing
flags. We've been using this in Debian already for the past few releases.
Making this change also encourages future maintainers towards "best practises".
Also take the opportunity to use the same method at all places in the file.

Author: anguslees

mk/rt.mk

@@ -253,7 +253,7 @@ COMPRT_AR_$(1) := $$(AR_$(1))
 # We chomp -Werror here because GCC warns about the type signature of
 # builtins not matching its own and the build fails. It's a bit hacky,
 # but what can we do, we're building libclang-rt using GCC ......
-COMPRT_CFLAGS_$(1) := $$(filter-out -Werror -Werror=*,$$(CFG_GCCISH_CFLAGS_$(1))) -std=c99
+COMPRT_CFLAGS_$(1) := $$(CFG_GCCISH_CFLAGS_$(1)) -Wno-error -std=c99
 
 # FreeBSD Clang's packaging is problematic; it doesn't copy unwind.h to
 # the standard include directory. This should really be in our changes to
@@ -361,7 +361,7 @@ $$(BACKTRACE_BUILD_DIR_$(1))/Makefile: $$(BACKTRACE_DEPS) $$(MKFILE_DEPS)
 	      CC="$$(CC_$(1))" \
 	      AR="$$(AR_$(1))" \
 	      RANLIB="$$(AR_$(1)) s" \
-	      CFLAGS="$$(CFG_GCCISH_CFLAGS_$(1):-Werror=) -fno-stack-protector" \
+	      CFLAGS="$$(CFG_GCCISH_CFLAGS_$(1)) -Wno-error -fno-stack-protector" \
 	      $(S)src/libbacktrace/configure --build=$(CFG_GNU_TRIPLE_$(CFG_BUILD)) --host=$(CFG_GNU_TRIPLE_$(1)))
 	$$(Q)echo '#undef HAVE_ATOMIC_FUNCTIONS' >> \
 	      $$(BACKTRACE_BUILD_DIR_$(1))/config.h