Workaround GCC 4.7+ optimizer bug by disabling optimization for the affected function

GCC 4.7+ incorrectly optimizes the GdipAddPathStringI() function, resulting in the value of r->Y being incorrect in the called GdipAddPathString() function.
By compiling with -O0 or adding a printf("Test") after r = &rect this doesn't happen.
Merge pull request #25 from mono/gcc-bug

Workaround GCC 4.7+ optimizer bug by disabling optimization for the affected function