json_reformat should link against libm

This is specific to qnx.

build error with qnx6.5.0

../yajl-2.0.1/lib/libyajl_s.a(yajl_gen.c.obj): In function yajl_gen_double':
yajl_gen.c:(.text+0x5b9): undefined reference to_DclassBuilding C object example/CMakeFiles/parse_config.dir/parse_config.c.obj
'
yajl_gen.c:(.text+0x5cf): undefined reference to `_Dclass'
cc: /opt/qnx650/host/linux/x86/usr/bin/ntox86-ld error 1
make[2]: *** [reformatter/json_reformat] Error 1

any platform that links does not link libm while linking libstdc

http://community.qnx.com/sf/discussion/do/listPosts/projects.toolchain/discussion.core_development_tools.topc10811?_pagenum=2