Makefile: add DISABLE_WERROR

Allow the user to disable -Werror to avoid the following build failure
with gcc 4.8 raised since version 0.6.0 and
e2a4ed1:

/home/buildroot/autobuild/instance-1/output-1/host/bin/arm-none-linux-gnueabi-gcc -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -I./src -std=gnu99 -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -Os -g0 -D_FORTIFY_SOURCE=1 -Werror=incompatible-pointer-types -Werror=implicit-function-declaration -Werror=int-conversion -fPIC  -c -o src/cborerrorstrings.pic.o src/cborerrorstrings.c
cc1: error: -Werror=incompatible-pointer-types: no option -Wincompatible-pointer-types
cc1: error: -Werror=int-conversion: no option -Wint-conversion

Fixes:
 - http://autobuild.buildroot.org/results/a9f/a9fe64c42bb96f9e7b4af3050464f6570c1c00fa

Signed-off-by: Fabrice Fontaine <[email protected]>

Author: ffontaine

Makefile

@@ -236,10 +236,15 @@ tag: distcheck
 .SECONDARY:
 
 cflags := $(CPPFLAGS) -I$(SRCDIR)src
-cflags += -std=gnu99 $(CFLAGS) \
+cflags += -std=gnu99 $(CFLAGS)
+
+ifneq ($(DISABLE_WERROR),1)
+cflags += \
 	-Werror=incompatible-pointer-types \
 	-Werror=implicit-function-declaration \
 	-Werror=int-conversion
+endif
+
 %.o: %.c
 	@test -d $(@D) || $(MKDIR) $(@D)
 	$(CC) $(cflags) $($(basename $(notdir $@))_CCFLAGS) -c -o $@ $<