fontconfig: Add --ignore-mtime option to fc-cache

:Release Notes:
Add a --ignore-mtime option to the fontconfig utility fc-cache.

:Detailed Notes:
Rather than storing the font directory mtime in the font cache file,
this sets a flag that instructs fontconfig not to discard the font
cache regardless of font directory mtime.

:Testing Performed:
Tested expected behavior with and without --ignore-mtime

:QA Notes:

:Issues Addressed:
[GF-46451] Make fontconfig's font-cache files as part of device's
           read-only filesystem
[GF-55668] CCC: fontconfig=webos1

Open-webOS-DCO-1.0-Signed-off-by: Martin Unsal <martin.unsal@lge.com>
Change-Id: If74f19aafa49a99f13ab1bc65f8834645bcc65a2
Reviewed-on: https://g2g.palm.com/4205
Reviewed-by: DCO Verification
Reviewed-by: Herb Kuta <herb.kuta@lge.com>
Tested-by: Herb Kuta <herb.kuta@lge.com>