Build gmp and mpfr libary on x86_32 only (fix #135)

By now the gmp-library works builds on x86/32bit only. The mpfr library
depends on gmp, so make both library dependent on the corresponding platform
variables. Look at the discussion of issue #135.

libports/lib/mk/gmp.inc

@@ -4,6 +4,9 @@ ifeq ($(wildcard $(GMP_DIR)),)
 REQUIRES += prepare_gmp
 endif
 
+# gmp depends on gmp-mpn which requires 32bit by now
+REQUIRES = x86_32
+
 include $(REP_DIR)/lib/import/import-gmp.mk
 
 LIBS += libc

libports/lib/mk/mpfr.mk

@@ -4,8 +4,8 @@ ifeq ($(wildcard $(MPFR_DIR)),)
 REQUIRES += prepare_mpfr
 endif
 
-# mpfr depends on gmp, which is only supported on x86 yet
-REQUIRES += x86
+# mpfr depends on gmp, which is only supported on x86_32 yet
+REQUIRES += x86_32
 
 LIBS      = libc gmp
 CC_OPT   += -DHAVE_STDARG -DHAVE_VA_COPY -DHAVE_INTTYPES_H