PATCH to use -Wno-format during stage1

[Jason Merrill <jason at redhat dot com>]

I find -Wformat warnings about unknown % codes when building with an 
older GCC to be mildly annoying noise; this patch avoids them by passing 
-Wno-format during stage 1.

Tested x86_64-pc-linux-gnu.  Is this OK for trunk?  Do you have another 
theory of how this should work?

commit c40b06619fc9ef74e4d4d8b299a6c77c6fb63df5
Author: Jason Merrill <jason@redhat.com>
Date:   Mon Oct 28 16:45:05 2013 -0400

    /
    	* Makefile.tpl (STAGE1_CONFIGURE_FLAGS): Pass
    	--disable-build-format-warnings.
    gcc/
    	* configure.ac (loose_warn): Add -Wno-format if
    	--disable-build-format-warnings.

diff --git a/Makefile.in b/Makefile.in
index 572b3d0..e0ba784 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -498,8 +498,10 @@ STAGE1_LANGUAGES = @stage1_languages@
 #   the last argument when conflicting --enable arguments are passed.
 # * Likewise, we force-disable coverage flags, since the installed
 #   compiler probably has never heard of them.
+# * We also disable -Wformat, since older GCCs don't understand newer %s.
 STAGE1_CONFIGURE_FLAGS = --disable-intermodule $(STAGE1_CHECKING) \
-	  --disable-coverage --enable-languages="$(STAGE1_LANGUAGES)"
+	  --disable-coverage --enable-languages="$(STAGE1_LANGUAGES)" \
+	  --disable-build-format-warnings
 
 STAGEprofile_CFLAGS = $(STAGE2_CFLAGS) -fprofile-generate
 STAGEprofile_TFLAGS = $(STAGE2_TFLAGS)
diff --git a/Makefile.tpl b/Makefile.tpl
index 3e187e1..65d070b 100644
--- a/Makefile.tpl
+++ b/Makefile.tpl
@@ -451,8 +451,10 @@ STAGE1_LANGUAGES = @stage1_languages@
 #   the last argument when conflicting --enable arguments are passed.
 # * Likewise, we force-disable coverage flags, since the installed
 #   compiler probably has never heard of them.
+# * We also disable -Wformat, since older GCCs don't understand newer %s.
 STAGE1_CONFIGURE_FLAGS = --disable-intermodule $(STAGE1_CHECKING) \
-	  --disable-coverage --enable-languages="$(STAGE1_LANGUAGES)"
+	  --disable-coverage --enable-languages="$(STAGE1_LANGUAGES)" \
+	  --disable-build-format-warnings
 
 STAGEprofile_CFLAGS = $(STAGE2_CFLAGS) -fprofile-generate
 STAGEprofile_TFLAGS = $(STAGE2_TFLAGS)
diff --git a/gcc/configure b/gcc/configure
index 1e7bcb6..ea91906 100755
--- a/gcc/configure
+++ b/gcc/configure
@@ -875,6 +875,7 @@ with_demangler_in_ld
 with_gnu_as
 with_as
 enable_largefile
+enable_build_format_warnings
 enable_werror_always
 enable_checking
 enable_coverage
@@ -1569,6 +1570,8 @@ Optional Features:
                           for creating source tarballs for users without
                           texinfo bison or flex
   --disable-largefile     omit support for large files
+  --disable-build-format-warnings
+                          don't use -Wformat while building GCC
   --enable-werror-always  enable -Werror despite compiler version
   --enable-checking[=LIST]
                           enable expensive run-time checks. With LIST, enable
@@ -6270,9 +6273,22 @@ fi
 # * C++11 narrowing conversions in { }
 # So, we only use -pedantic if we can disable those warnings.
 
+# In stage 1, disable -Wformat warnings from old GCCs about new % codes
+# Check whether --enable-build-format-warnings was given.
+if test "${enable_build_format_warnings+set}" = set; then :
+  enableval=$enable_build_format_warnings;
+else
+  enable_build_format_warnings=yes
+fi
+
+if test $enable_build_format_warnings = no; then :
+  wf_opt=-Wno-format
+else
+  wf_opt=
+fi
 loose_warn=
 save_CFLAGS="$CFLAGS"
-for real_option in -W -Wall -Wno-narrowing -Wwrite-strings -Wcast-qual; do
+for real_option in -W -Wall -Wno-narrowing -Wwrite-strings -Wcast-qual $wf_opt; do
   # Do the check with the no- prefix removed since gcc silently
   # accepts any -Wno-* option on purpose
   case $real_option in
@@ -17897,7 +17913,7 @@ else
   lt_dlunknown=0; lt_dlno_uscore=1; lt_dlneed_uscore=2
   lt_status=$lt_dlunknown
   cat > conftest.$ac_ext <<_LT_EOF
-#line 17900 "configure"
+#line 17916 "configure"
 #include "confdefs.h"
 
 #if HAVE_DLFCN_H
@@ -18003,7 +18019,7 @@ else
   lt_dlunknown=0; lt_dlno_uscore=1; lt_dlneed_uscore=2
   lt_status=$lt_dlunknown
   cat > conftest.$ac_ext <<_LT_EOF
-#line 18006 "configure"
+#line 18022 "configure"
 #include "confdefs.h"
 
 #if HAVE_DLFCN_H
diff --git a/gcc/configure.ac b/gcc/configure.ac
index 5e686db..3d3b26b 100644
--- a/gcc/configure.ac
+++ b/gcc/configure.ac
@@ -326,8 +326,14 @@ GCC_STDINT_TYPES
 # * C++11 narrowing conversions in { }
 # So, we only use -pedantic if we can disable those warnings.
 
+# In stage 1, disable -Wformat warnings from old GCCs about new % codes
+AC_ARG_ENABLE(build-format-warnings,
+  AS_HELP_STRING([--disable-build-format-warnings],[don't use -Wformat while building GCC]),
+  [],[enable_build_format_warnings=yes])
+AS_IF([test $enable_build_format_warnings = no],
+      [wf_opt=-Wno-format],[wf_opt=])
 ACX_PROG_CC_WARNING_OPTS(
-	m4_quote(m4_do([-W -Wall -Wno-narrowing -Wwrite-strings -Wcast-qual])), [loose_warn])
+	m4_quote(m4_do([-W -Wall -Wno-narrowing -Wwrite-strings -Wcast-qual $wf_opt])), [loose_warn])
 ACX_PROG_CC_WARNING_OPTS(
 	m4_quote(m4_do([-Wstrict-prototypes -Wmissing-prototypes])),
 	[c_loose_warn])