Add support for RHEL/centos to installdeps. There is no SFML package.

Add some string compatibility stuff so that everything builds on wx 2.8
unicode builds.

TODO: add travis slaves for 2.8, with and without unicode

Disable the game frame spacer code on 2.8 because it does not work (the
frame does not expand.)

cmake improvements:

- set ENABLE_LTO=OFF for Debug builds by default

- set ENABLE_LINK=OFF if SFML cannot be found and continue

- use better logic for finding wx utilities wx-config and wxrc

Signed-off-by: Rafael Kitover <rkitover@gmail.com>
Closes #395
Closes #297