Allow the environment variable RCC to set before running configure to
allow for unusual paths for Qt's rcc program (FreeBSD ports puts it in
/usr/local/lib/qt5/bin).  This matches the existing functionality of the
MOC and UIC variables/programs.

Change-Id: I885d61bce54d9e893e0c2fc496a9ff767a096ae6
Reviewed-on: https://code.wireshark.org/review/5139
Petri-Dish: Stephen Fisher <stephenfisher@centurylink.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Stephen Fisher <stephenfisher@centurylink.net>