crt: Improve the cpp parameters for preprocessing def files

Use -undef to avoid any compiler specific predefined definitions,
and use -P to skip the line markers - removing the need for the extra
sed command.

The -w command to silence warnings is necessary, since otherwise
there are unnecessary warnings like these:
lib64/msvcrt.def.in:66:37: warning: missing terminating ' character
 ; const  __non_rtti_object::`vftable'

Signed-off-by: Martin Storsjö <martin@martin.st>

mingw-w64-crt/Makefile.am

diff --git a/mingw-w64-crt/Makefile.am b/mingw-w64-crt/Makefile.am
index 1600ac2..b6417ec 100644
--- a/mingw-w64-crt/Makefile.am
+++ b/mingw-w64-crt/Makefile.am
@@ -1239,7 +1239,7 @@
 # ##########
 
 %.def: %.def.in
-	$(MKDIR_P) $(@D) && $(CPP) -x c $< -Wp,-w -I$(top_srcdir)/def-include | $(SED) 's/^#/;/' > $@
+	$(MKDIR_P) $(@D) && $(CPP) -x c $< -Wp,-w -undef -P -I$(top_srcdir)/def-include > $@
 
 # Don't compile these, but install as is:
 #

mingw-w64-crt/Makefile.in

diff --git a/mingw-w64-crt/Makefile.in b/mingw-w64-crt/Makefile.in
index eaa8a2c..2464559 100644
--- a/mingw-w64-crt/Makefile.in
+++ b/mingw-w64-crt/Makefile.in
@@ -49240,7 +49240,7 @@
 # ##########
 
 %.def: %.def.in
-	$(MKDIR_P) $(@D) && $(CPP) -x c $< -Wp,-w -I$(top_srcdir)/def-include | $(SED) 's/^#/;/' > $@
+	$(MKDIR_P) $(@D) && $(CPP) -x c $< -Wp,-w -undef -P -I$(top_srcdir)/def-include > $@
 
 dist-hook:
 	find $(top_distdir) -name ".svn" -delete