Add USE_INTERNAL_LIBS, a default for USE_INTERNAL_*

Linux distributions that want to link dependencies externally will
generally want to link (almost) every dependency externally; similarly,
minimal-dependency builds that want to use the embedded copies of
dependencies will generally want to do so for (almost) every dependency.
Make it easier to choose one of those by setting USE_INTERNAL_LIBS=0
or USE_INTERNAL_LIBS=1, respectively.

The default can still be overridden per-dependency; for instance,
"make USE_INTERNAL_LIBS=0 USE_INTERNAL_OPUS=1" will use the system
version of everything except Opus.
Centralize checks for Freetype, with a user override

This lets us find a library in a non-standard library directory
(via -L in the pkg-config metadata), and allows overrides similar to
the Autoconf convention, e.g.

    make FREETYPE_CFLAGS=-I/opt/freetype/include \
        FREETYPE_LIBS="-L/opt/freetype/lib -lfreetype"

If pkg-config didn't work, assume that Freetype is in the default
location.
Allow libcurl to be in a non-standard location on all platforms

We didn't add CURL_CFLAGS to CLIENT_CFLAGS on all platforms, and didn't
use CURL_LIBS at all, so if "pkg-config --libs" returned "-L... -lcurl"
or even "/.../libcurl.a", it wouldn't work.
Allow OpenAL to be in a non-standard location on all platforms

Similar to libcurl, we didn't use OPENAL_LIBS and assumed it was
always "-lopenal".
Allow system libjpeg to be in a non-standard location

It doesn't have pkg-config metadata (at least on Debian), so if the
user doesn't override it, assume normal system paths.
Find system zlib via user override, pkg-config or in standard locations
Look for system Vorbis, Opus and Ogg via pkg-config

As usual, the order of precedence is: user override, pkg-config,
or assume they're in standard locations.

In particular, Opus isn't in the default search path on Debian.
Find Speex via user override, pkg-config or default search path
Add support for x86_64 cross compiling in Travis CI
Add misc/msvc12/ to support Microsoft Visual Studio 2012 Express Edition
Merge pull request #63 from kungfooman/visualstudio

Add misc/msvc11/ to support Microsoft Visual Studio 2012 Express Edition
Add MSVC SDL 1.2.15 Revision 8040 import libs
OpenGL2: Remove sRGB support, replace with gamma cvars.
Rebuilt MSVC SDL 1.2.15 Revision 8040 import libs
OpenGL2: Remove misleading documentation for gamma cvars.
Fix cg.skulltrails out of bounds access in Team Arena Harvester mode

In Team Arena's Harvester mode, players corrupt your memory from beyond
the grave. Gib the players to stop the corruption!

CG_PlayerTokens is called for player entities, including corpses.
The entity number is used for the index in cg.skulltrails which only has
MAX_CLIENTS elements. This results in incorrect memory being overwritten
for corpse entities (as the entity number is >= MAX_CLIENTS).

So limit skull trails to valid entities (entity number < MAX_CLIENTS).
Make R_LerpTag return qfalse if MDR tag does not exist

Elite Force SDK has trap_R_LerpTag return void, so this shouldn't cause issues for iostvef.
Allows new games to check if a tag exists in a MDR model.
Switched runtime library to Multithreaded DLL to fix build on VS2012.
Merge pull request #69 from RobertoEstrada/master

Switched runtime library to Multithreaded DLL to fix build on VS2012.
Don't install libvorbis-dev on Travis

The in-tree copy of vorbis is used. No reason to install the package anymore.