Stay compatible with old gmake that did not support undefine

Makefile.system

@@ -6,7 +6,7 @@
 INCLUDED = 1
 
 ifndef TOPDIR
-TOPDIR = .
+TOPDIR = . 
 endif
 
  # If ARCH is not set, we use the host system's architecture for getarch compile options.
@@ -252,30 +252,22 @@ DUMMY := $(shell $(MAKE) -C $(TOPDIR) -f Makefile.prebuild CC="$(CC)" FC="$(FC)"
 ifndef TARGET_CORE
 include $(TOPDIR)/Makefile.conf
 else
-ifdef HAVE_NEON
-undefine HAVE_NEON
-endif
-ifdef HAVE_VFP
-undefine HAVE_VFP
-endif
-ifdef HAVE_VFPV3
-undefine HAVE_VFPV3
-endif
-ifdef HAVE_VFPV4
-undefine HAVE_VFPV4
-endif
-undefine HAVE_MMX
-undefine HAVE_SSE
-undefine HAVE_SSE2
-undefine HAVE_SSE3
-undefine HAVE_SSSE3
-undefine HAVE_SSE4_1
-undefine HAVE_SSE4_2
-undefine HAVE_SSE4A
-undefine HAVE_SSE5
-undefine HAVE_AVX
-undefine HAVE_AVX2
-undefine HAVE_FMA3
+HAVE_NEON=
+HAVE_VFP=
+HAVE_VFPV3=
+HAVE_VFPV4=
+HAVE_MMX=
+HAVE_SSE=
+HAVE_SSE2=
+HAVE_SSE3=
+HAVE_SSSE3=
+HAVE_SSE4_1=
+HAVE_SSE4_2=
+HAVE_SSE4A=
+HAVE_SSE5=
+HAVE_AVX=
+HAVE_AVX2=
+HAVE_FMA3=
 include $(TOPDIR)/Makefile_kernel.conf
 endif