build: improve build-from-git for older GCCs

configure.ac: Bump GCC version from 4.6 to 11.2 when deciding whether
to default to enabling GCC warnings when --enable-gcc-warnings is not
specified, as older GCCs can generate too many false alarms.  From
a suggestion by Christian Schoenebeck.

configure.ac

@@ -158,7 +158,7 @@ AC_ARG_ENABLE([gcc-warnings],
    gl_gcc_warnings=$enableval],
   [gl_gcc_warnings=no
    if test -d "$srcdir"/.git; then
-     gl_GCC_VERSION_IFELSE([4], [6], [gl_gcc_warnings=yes])
+     gl_GCC_VERSION_IFELSE([11], [2], [gl_gcc_warnings=yes])
    fi]
 )