Enable all warnings permanently & default to -Werror for GIT builds

Given that we auto-detect whether each -Wxxxx flag is supported by
GCC, and we are warning-free and use automake silent rules, there
is no compelling reason to allow compile warnings to be disabled.

Replace the --enable-compile-warnings flag with a simpler
--enable-werror flag, which defaults to 'yes' if building
from GIT, or 'no' if building from tar.gz

This helps ensure that everyone writing patches for libvirt will
take care to fix their warning problems before submitting for
review

* autobuild.sh: Force -Werror
* configure.ac: Update for LIBVIRT_COMPILE_WARNINGS macro change
* m4/virt-compile-warnings.m4: Permanently enable all warnings,
  auto-enable Werror for GIT builds

autobuild.sh

@@ -20,7 +20,7 @@ cd build
 ../autogen.sh --prefix="$AUTOBUILD_INSTALL_ROOT" \
   --enable-test-coverage \
   --disable-nls \
-  --enable-compile-warnings=error
+  --enable-werror
 
 # If the MAKEFLAGS envvar does not yet include a -j option,
 # add -jN where N depends on the number of processors.
@@ -74,7 +74,7 @@ if [ -x /usr/bin/i686-pc-mingw32-gcc ]; then
     --build=$(uname -m)-pc-linux \
     --host=i686-pc-mingw32 \
     --prefix="$AUTOBUILD_INSTALL_ROOT/i686-pc-mingw32/sys-root/mingw" \
-    --enable-compile-warnings=error \
+    --enable-werror \
     --without-libvirtd \
     --without-python
 

configure.ac

@@ -106,7 +106,7 @@ VERSION_SCRIPT_FLAGS=-Wl,--version-script=
     VERSION_SCRIPT_FLAGS="-Wl,-M -Wl,"
 AC_MSG_RESULT([$VERSION_SCRIPT_FLAGS])
 
-LIBVIRT_COMPILE_WARNINGS([maximum])
+LIBVIRT_COMPILE_WARNINGS
 
 AC_MSG_CHECKING([for CPUID instruction])
 AC_COMPILE_IFELSE([AC_LANG_PROGRAM(
@@ -2820,7 +2820,7 @@ AC_MSG_NOTICE([])
 AC_MSG_NOTICE([Miscellaneous])
 AC_MSG_NOTICE([])
 AC_MSG_NOTICE([        Debug: $enable_debug])
-AC_MSG_NOTICE([     Warnings: $enable_compile_warnings])
+AC_MSG_NOTICE([  Use -Werror: $set_werror])
 AC_MSG_NOTICE([Warning Flags: $WARN_CFLAGS])
 AC_MSG_NOTICE([     Readline: $lv_use_readline])
 AC_MSG_NOTICE([       Python: $with_python])

m4/virt-compile-warnings.m4

@@ -7,16 +7,15 @@ AC_DEFUN([LIBVIRT_COMPILE_WARNINGS],[
     dnl More compiler warnings
     dnl ******************************
 
-    AC_ARG_ENABLE(compile-warnings,
-                  [AC_HELP_STRING([--enable-compile-warnings=@<:@no/yes/error@:>@],
-                                 [Turn on compiler warnings])],,
-                  [enable_compile_warnings="m4_default([$1],[yes])"])
-
-    case "$enable_compile_warnings" in
-    no)
-        try_compiler_flags=""
-        ;;
-    yes|minimum|maximum|error)
+    AC_ARG_ENABLE([werror],
+                  AS_HELP_STRING([--enable-werror], [Use -Werror (if supported)]),
+                  [set_werror="$enableval"],
+                  [if test -d $srcdir/.git; then
+                     is_git_version=true
+                     set_werror=yes
+                   else
+                     set_werror=no
+                   fi])
 
     # List of warnings that are not relevant / wanted
 
@@ -131,15 +130,10 @@ AC_DEFUN([LIBVIRT_COMPILE_WARNINGS],[
     gl_WARN_ADD([-Wno-suggest-attribute=pure])
     gl_WARN_ADD([-Wno-suggest-attribute=const])
 
-        if test "$enable_compile_warnings" = "error"
+    if test "$set_werror" = "yes"
     then
       gl_WARN_ADD([-Werror])
     fi
-        ;;
-    *)
-        AC_MSG_ERROR(Unknown argument '$enable_compile_warnings' to --enable-compile-warnings)
-        ;;
-    esac
 
     WARN_LDFLAGS=$WARN_CFLAGS
     AC_SUBST([WARN_CFLAGS])