GC_PTR is declared in guile (libguile/bdw-gc.h) under the condition #if (defined GC_VERSION_MAJOR) && (GC_VERSION_MAJOR >= 7) but libgc-7.2/gc.h declares it "for backward compatibility". Resulting in a redeclaration error.
This effects the head of master, stable-2.0 and the latest release 2.0.5
