Makefile: add DISABLE_WERROR
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
https://github.com/intel/tinycbor/commit/e2a4ed135c4d9101c4df83f2dd033cd249b6ef07:

/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 <7d88633730b8b85c1516e612893df72eae9cd341@gmail.com>

intel/tinycbor,intel/tinycbor,intel/tinycbor,intel/tinycbor