configure: fix expanding system commands in qmake parser with GNU awk < 4

qmake variables using $$system() were incorrectly parsed by the custom
qmake parser in the configure script, when using GNU awk 3.1.8 or
earlier. They are parsed correctly with GNU awk 4 or mawk.

This was occurring with such an assignement (from an extra mkspecs file):
    QMAKE_CC                = $$system($$CMD QMAKE_CC 2>/dev/null)

The custom qmake parser in the configure script first attempts to
expand $$UPPERCASE variables, before running $$system(), using this:
    match(value, /\$\$(\{[_A-Z0-9.]+\}|[_A-Z0-9.]+)/)

But when using non-ASCII locales with GNU awk 3.1.8 or earlier,
$$system was expanded (to an empty string) because these earlier awk
versions match lowercase letters for the [A-Z] regexp, which is
traditionally used to match uppercase letters.

This behavior has been changed in GNU awk 4.0.0, which only matches
uppercase letters for [A-Z] by default. A workaround for earlier GNU
awk versions is to run awk with the C locale.

See GNU awk NEWS "Changes from 3.1.8 to 4.0.0":
25. Gawk now treats ranges of the form [d-h] as if they were in the C
    locale, no matter what kind of regexp is being used, and even if
    --posix.  The latest POSIX standard allows this, and the documentation
    has been updated.  Maybe this will stop all the questions about
    [a-z] matching uppercase letters.
    THIS CHANGES BEHAVIOR!!!!

See also gawk.info "A.7 Regexp Ranges and Locales: A Long Sad Story"

Change-Id: Ibb3eb28738c3e77d496c634e1f5c9f630957e730
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>