Do not suggest to use gas-preprocessor if using it would break compilation.

gas-preprocessor does not work on ppc with pic and on ppc64.

configure

@@ -3865,7 +3865,9 @@ elif enabled ppc; then
 
     # AltiVec flags: The FSF version of GCC differs from the Apple version
     if enabled altivec; then
-        nogas=warn
+        if ! enabled_any pic ppc64; then
+            nogas=warn
+        fi
         check_cflags -maltivec -mabi=altivec &&
         { check_header altivec.h && inc_altivec_h="#include <altivec.h>" ; } ||
         check_cflags -faltivec