build: make strerror() override-able

If we want to override `strerror()` in libjson-c
to make tests consistent across platforms, we
need to do it build-wide as configure/build
option.

Apple linkers make it really hard to override functions
at link-time, and this seems to be locked down on travis-ci.org
[ for security reasons I assume ].
While I got it to work locally, it did not work
when running on travis.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

Makefile.am

@@ -53,6 +53,12 @@ libjson_c_la_SOURCES = \
 	printbuf.c \
 	random_seed.c
 
+if ENABLE_STRERROR_OVERRIDE
+libjson_cinclude_HEADERS+= \
+	strerror_override.h
+libjson_c_la_SOURCES+= \
+	strerror_override.c
+endif
 
 distclean-local:
 	-rm -rf $(testsubdir)