stable/build: Do not convert warnings to error

During skiboot build, by default we convert all warnings to error.
Because of this sometime skiboot stable branch fails to build on
modern compiler. And we endup backporting build failure fixes to
stable branches.

Hence lets disable `-Werror` on skiboot stable branches (tagged version).

Suggested-by: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Vasant Hegde <hegdevasant@linux.vnet.ibm.com>
Reviewed-by: Dan Horák <dan@danny.cz>
Signed-off-by: Oliver O'Halloran <oohall@gmail.com>

Makefile.main

@@ -31,8 +31,17 @@ CWARNS := -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs \
 	  -Wno-pointer-sign -Wextra -Wno-sign-compare \
 	  -Wmissing-prototypes -Wmissing-declarations \
 	  -Wwrite-strings -Wcast-align \
-	  -Winit-self \
-	  -Werror
+	  -Winit-self
+
+# Do not convert warnings to error on tagged/released version
+GIT_DIR ?= $(shell test -e $(SRC)/.git && echo 1 || echo 0)
+ifeq ($(GIT_DIR),1)	# Building from git tree
+GIT_SHA ?= $(shell cd $(SRC); git describe --tags 2>/dev/null)
+GIT_TAG ?= $(shell cd $(SRC); git tag -l 2>/dev/null | grep -x ${GIT_SHA} >/dev/null 2>&1 && echo 1 || echo 0)
+ifneq ($(GIT_TAG),1)	# Development branch
+CWARNS += -Werror
+endif
+endif
 
 # Host tools and options
 HOSTCC=gcc