build: --enable-gcc-warnings: disable some new warnings

* configure.ac: Disable some new warning options pulled in via
an update to gnulib's manywarnings module: -Wformat-nonliteral,
-Wunsuffixed-float-constants, -Wdouble-promotion.

configure.ac

@@ -121,6 +121,8 @@ if test "$gl_gcc_warnings" = yes; then
   gl_WARN_ADD([-Wno-unused-parameter]) # Too many warnings for now
   gl_WARN_ADD([-Wsuggest-attribute=const])
   gl_WARN_ADD([-Wsuggest-attribute=noreturn])
+  gl_WARN_ADD([-Wno-format-nonliteral])
+  gl_WARN_ADD([-Wno-unsuffixed-float-constants])
 
   # Enable this warning only with gcc-4.7 and newer.  With 4.6.1 20110824,
   # it suggests test.c's advance function may be pure, even though it
@@ -151,6 +153,9 @@ if test "$gl_gcc_warnings" = yes; then
   nw="$nw -Wunused-macros"
   nw="$nw -Wmissing-prototypes"
   nw="$nw -Wold-style-definition"
+  # FIXME: it may be easy to remove this, since it affects only one file:
+  # the snprintf call at ftoastr.c:132.
+  nw="$nw -Wdouble-promotion"
   # FIXME: remove/reenable the following two, once gnulib is adjusted.
   nw="$nw -Wsuggest-attribute=const"
   nw="$nw -Wsuggest-attribute=pure"