great idea, but let's use the already existing XCFLAGS substitution, enable the stuff by default and just name it with_a4 because I think this is already clear that it is just the default and it can be overridden explicitly

ghostscript/ghostscript.spec

@@ -46,7 +46,7 @@ Release:      20031011
 
 #   package options
 %option       with_x11     yes
-%option       with_def_a4  no
+%option       with_a4      yes
 
 #   list of sources
 Source0:      ftp://mirror.cs.wisc.edu/pub/mirrors/ghost/AFPL/gs%{V_comp}/ghostscript-%{V_real}.tar.bz2
@@ -102,6 +102,9 @@ AutoReqProv:  no
 %else
     x11_inc=""
     x11_lib=""
+%endif
+%if "%{with_a4}" == "yes"
+    mtcflags="$mtcflags -DA4"
 %endif
     %{l_shtool} subst \
          -e "s;^STDLIBS=-lpthread -lm;STDLIBS=$mtldflags -lm;" \
@@ -117,11 +120,7 @@ AutoReqProv:  no
          -e 's;DEVICE_DEVS=.*$;DEVICE_DEVS=;' \
          src/unix-gcc.mak
 %endif
-%if "%{with_def_a4}" == "yes"
-    %{l_make} %{l_mflags} -f src/unix-gcc.mak XCFLAGS="-DA4"
-%else
     %{l_make} %{l_mflags} -f src/unix-gcc.mak
-%endif
 
 %install
     rm -rf $RPM_BUILD_ROOT