Clear CFLAGS and LDFLAGS to build the tests, in case the main Makefile
passes some flags that aren't handled by tcc (we are not compiling tcc
here, we are using tcc to compile the tests).
Prevent libtcc1.a to be compiled with -fstack-protector-strong, so that
linking with tcc doesn't fail because symbol '__stack_chk_fail_local' is
not present in libtcc1.a. This is useful only if the CFLAGS passed from
the main Makefile contain this flag.
Matteo Cypriani