Avoid CFLAGS being set by accident

When no CFLAGS are passed into autogen.sh,
f104b3ca "tdf#72987 run firebird test for little
endian only for now" (introducing AC_C_BIGENDIAN before AC_PROG_CC) caused
CFLAGS in config_host.mk to accidentally be set to -g -O2 (instead of being left
undefined).

Change-Id: I639ce74b468e7fb25a6bda97346c7f97d6b829aa
Reviewed-on: https://gerrit.libreoffice.org/67125
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

configure.ac

@@ -803,13 +803,6 @@ haiku*)
     ;;
 esac
 
-if test "$_os" != "WINNT"; then
-AC_C_BIGENDIAN([ENDIANNESS=big], [ENDIANNESS=little])
-else
-ENDIANNESS=little
-fi
-AC_SUBST(ENDIANNESS)
-
 if test "$_os" = "Android" ; then
     # Verify that the NDK and SDK options are proper
     if test -z "$with_android_ndk"; then
@@ -925,7 +918,7 @@ my_original_CXXFLAGS=$CXXFLAGS
 my_original_CPPFLAGS=$CPPFLAGS
 
 dnl The following checks for gcc, cc and then cl (if it weren't guarded for win32)
-dnl Needs to precede the AC_SEARCH_LIBS call below, which apparently calls
+dnl Needs to precede the AC_C_BIGENDIAN and AC_SEARCH_LIBS calls below, which apparently call
 dnl AC_PROG_CC internally.
 if test "$_os" != "WINNT"; then
     # AC_PROG_CC sets CFLAGS to -g -O2 if not set, avoid that
@@ -934,6 +927,13 @@ if test "$_os" != "WINNT"; then
     CFLAGS=$save_CFLAGS
 fi
 
+if test "$_os" != "WINNT"; then
+AC_C_BIGENDIAN([ENDIANNESS=big], [ENDIANNESS=little])
+else
+ENDIANNESS=little
+fi
+AC_SUBST(ENDIANNESS)
+
 if test $_os != "WINNT"; then
     save_LIBS="$LIBS"
     AC_SEARCH_LIBS([dlsym], [dl],