[libgc] Eliminate warning about redefinition of GC_WIN32_THREADS

Makefile defines GC_WIN32_THREADS as 1 but gc_config_macros was
redefining as defined but with no value.

Signed-off-by: Alex J Lennon <ajlennon@dynamicdevices.co.uk>
Merge pull request #1378 from DynamicDevices/ajl/warnings

[libgc] Eliminate warning about redefinition of GC_WIN32_THREADS