Adding Central config.cache options

Adding Central config.cache options

The following changes allow for use of a central configure cache
file.  This speeds up configuration of packages.
Its use is configurable at the top level (BR2_CONFIG_CACHE - default n).
Old style makefiles can use it if they use the following MACRO in makefiles:
$(AUTO_CONFIGURE_TARGET) see my change to directfb.mk.
New style Autotools.in will use it if you set the global option.
However you can enable the global option and on a per package overrule it by doing
the following: $(PKGNAME)_USE_CONFIG_CACHE = NO see fontconfig.mk for an example
of this.
Finally I have removed a few config variable settings which indicated no CXX compiler
as this is wrong and breaks the build when using this central cache.

 Config.in                        |    8 ++++++++
 package/Makefile.autotools.in    |    5 ++++-
 package/Makefile.in              |   28 +++++++++++++++++++++++++++-
 package/atk/atk.mk               |    2 +-
 package/directfb/directfb.mk     |    7 +------
 package/fontconfig/fontconfig.mk |    3 +++
 package/libglib2/libglib2.mk     |    2 +-
 package/libgtk2/libgtk2.mk       |    1 -
 8 files changed, 45 insertions(+), 11 deletions(-)

I would appreciate feedback on this change (I have been testing for 2-3 weeks)
But I can never test all cases!  If you enable the BR2_CONFIG_CACHE option some
Makefile.autotools.in based packages may now break - I cannot build them all.
In this case you may need to remove config options that are being hardcoded all
over the place (like gtk saying we have 2 CXX compiler) or disable the use
of CONFIG CACHE file like I have done in fontconfig.
I can build all packages required to get WebKit on DirectFB up and running
and it runs fine.
I will try to resolve any issues this creates as fast as I can.

Signed-off-by: Daniel Laird <daniel.j.laird@nxp.com>