Use only GCC-specific flags when compiling with GCC

[dago]

Hi,

these are some adjustments on the autoconf-foo that enhance the compatibility with non-GCC compilers.

Best regards -- Dago

[hawicz]

Why did you drop AC_C_CONST and AC_TYPE_SIZE_T?

[dago]

I only moved the two lines upwards, I can't remember the reason any more, probably because the manual suggested to have these tests early and before the header checks.

[hawicz]

The use of gcc specific flags on non-gcc compilers should be fixed by changeset 80c1f69
For anything else that's needed, please submit a separate pull request. However, munging the scripts to support lackluster shells leaves a bad taste in my mouth. If you can instead arrange for the tests to be executed with a fully working shell I'd be much happier to merge the change.