Fix hard-coded call to gcc in GCC version check

FreeBSD 10 does not have gcc by default, but the clang compiler does support GCC
features. Replaced the hard-coded gcc command with $CC.
Tested on FreeBSD 10, Ubuntu 12.04.4 and Mac OSX 10.9.3
Merge branch 'fix_gcc_version_check' of https://github.com/alistairking/libtrace into develop
Fixed broken -fvisibility check in configure

 * Added a m4 script that does this properly rather than our hax
   gcc version check.
 * Tidied up CFLAGS in configure so we aren't adding the same sets of
   flags multiple times
 * Created a wandio_internal.h file for storing global variables that
   shouldn't be made public

Thanks to Alistair King, whose patch to try and make this work for
non-gcc systems brought my attention to just how broken this was :)
Forgot to update lzo to include new wandio header
Merge branch 'gccvisibility' into develop