Change manywarnings usage to be more like coreutils

* configure.ac: Treat --enable-gcc-warnings more like coreutils does.
This mostly just migrates coreutils changes into this file.

configure.ac

@@ -44,22 +44,30 @@ gl_INIT
 AM_PROG_AR
 
 AC_ARG_ENABLE([gcc-warnings],
-  [AS_HELP_STRING([--enable-gcc-warnings],
-                  [turn on lots of GCC warnings (for developers)])],
+  [AS_HELP_STRING([--enable-gcc-warnings@<:@=TYPE@:>@],
+    [control generation of GCC warnings.  The TYPE 'no' disables
+     warnings (default for non-developer builds); 'yes' generates
+     cheap warnings if available;
+     'expensive' in addition generates expensive-to-compute warnings
+     if available.])],
   [case $enableval in
-     yes|no) ;;
+     no|yes|expensive) ;;
      *)      AC_MSG_ERROR([bad value $enableval for gcc-warnings option]) ;;
    esac
-   gl_gcc_warnings=$enableval],
-  [gl_gcc_warnings=no]
+   gl_gcc_warnings=$enableval]
 )
 
-if test "$gl_gcc_warnings" = yes; then
+if test $gl_gcc_warnings != no; then
   gl_WARN_ADD([-Werror], [WERROR_CFLAGS])
   AC_SUBST([WERROR_CFLAGS])
 
-  nw=
+  ew=
+  AS_IF([test $gl_gcc_warnings != expensive],
+    [# -fanalyzer and related options slow GCC considerably.
+     ew="$ew -fanalyzer -Wno-analyzer-malloc-leak"])
+
   # This, $nw, is the list of warnings we disable.
+  nw=$ew
   nw="$nw -Wdeclaration-after-statement" # too useful to forbid
   nw="$nw -Waggregate-return"       # anachronistic
   nw="$nw -Wlong-long"              # C90 is anachronistic (lib/gethrxtime.h)
@@ -74,40 +82,28 @@ if test "$gl_gcc_warnings" = yes; then
   nw="$nw -Wunreachable-code"       # Too many warnings for now
   nw="$nw -Wpadded"                 # Our structs are not padded
   nw="$nw -Wredundant-decls"        # openat.h declares e.g., mkdirat
-  nw="$nw -Wlogical-op"             # any use of fwrite provokes this
   nw="$nw -Wformat-nonliteral"      # who.c and pinky.c strftime uses
-  nw="$nw -Wvla"                    # warnings in gettext.h
   nw="$nw -Wnested-externs"         # use of XARGMATCH/verify_function__
   nw="$nw -Wswitch-enum"            # Too many warnings for now
   nw="$nw -Wswitch-default"         # Too many warnings for now
   nw="$nw -Wstack-protector"        # not worth working around
-  nw="$nw -Wmissing-format-attribute" # not worth working around in patch
-  nw="$nw -Wsuggest-attribute=format" # warns about util.c
-  # things to fix soon:
-  nw="$nw -Wshadow"
-  nw="$nw -Wstrict-overflow"
-  nw="$nw -Wunsafe-loop-optimizations"
-#  nw="$nw -Wfloat-equal"            # sort.c, seq.c
-#  nw="$nw -Wmissing-format-attribute" # copy.c
   nw="$nw -Winline"                  # It's OK for a compiler to not inline.
-  # ?? -Wstrict-overflow
+
+  # Using -Wstrict-overflow is a pain, but the alternative is worse.
+  # For an example, see the code that provoked this report:
+  # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=33498
+  # Code like that still infloops with gcc-4.6.0 and -O2.  Scary indeed.
 
   gl_MANYWARN_ALL_GCC([ws])
   gl_MANYWARN_COMPLEMENT([ws], [$ws], [$nw])
   for w in $ws; do
     gl_WARN_ADD([$w])
   done
-  gl_WARN_ADD([-Wno-missing-field-initializers]) # We need this one
   gl_WARN_ADD([-Wno-sign-compare])     # Too many warnings for now
   gl_WARN_ADD([-Wno-unused-parameter]) # Too many warnings for now
-
-  # In spite of excluding -Wlogical-op above, it is enabled, as of
-  # gcc 4.5.0 20090517, and it provokes warnings in cat.c, dd.c, truncate.c
-  gl_WARN_ADD([-Wno-logical-op])
+  gl_WARN_ADD([-Wno-format-nonliteral])
 
   gl_WARN_ADD([-fdiagnostics-show-option])
-  gl_WARN_ADD([-funit-at-a-time])
-  gl_WARN_ADD([-Wno-format-nonliteral])
 
   AC_SUBST([WARN_CFLAGS])
 
@@ -123,22 +119,19 @@ if test "$gl_gcc_warnings" = yes; then
 
   # We use a slightly smaller set of warning options for lib/.
   # Remove the following and save the result in GNULIB_WARN_CFLAGS.
-  nw=
+  nw=$ew
   nw="$nw -Wuninitialized"
   nw="$nw -Wunused-macros"
   nw="$nw -Wmissing-prototypes"
   nw="$nw -Wold-style-definition"
   gl_MANYWARN_COMPLEMENT([GNULIB_WARN_CFLAGS], [$WARN_CFLAGS], [$nw])
   AC_SUBST([GNULIB_WARN_CFLAGS])
-
-  # For gnulib-tests, the set is slightly smaller still.
-  nw=
-  nw="$nw -Wstrict-prototypes"
-  gl_MANYWARN_COMPLEMENT([GNULIB_TEST_WARN_CFLAGS],
-                         [$GNULIB_WARN_CFLAGS], [$nw])
-  AC_SUBST([GNULIB_TEST_WARN_CFLAGS])
 fi
 
+# Ensure VLAs are not used.
+# Note -Wvla is implicitly added by gl_MANYWARN_ALL_GCC
+AC_DEFINE([GNULIB_NO_VLA], [1], [Define to 1 to disable use of VLAs])
+
 AC_TYPE_MODE_T
 AC_TYPE_OFF_T