Fix compilation of testgearctl using CFLAGS

Using make CFLAGS would break the CFLAGS of lua-5.2.3. This is avoided
by letting the CFLAGS in the lua makefile override any externally
applied CFLAGS.

This allows supporting compilation using testgear headers installed in
nonstandard include directory. Eg.:

make CFLAGS=-I$HOME/opt/testgear/include