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 vorbis 1.3.4 build support
Upgrade from libcurl 7.15.5 (2006-08-07) to 7.35.0 (2014-01-29)

Note: This uses the generic curlbuild.h because it is used by multiple platforms.
A system curl would have a configure generated one.

The libraries for win32 and win64 were cross compiled with these options:

  --disable-shared --enable-static --disable-ares --enable-http \
  --enable-ftp --disable-ldap --disable-ldaps --disable-rtsp \
  --disable-dict --disable-file --disable-telnet --disable-tftp \
  --disable-pop3 --disable-imap --disable-smtp --disable-gopher \
  --enable-ipv6 --without-ssl \
  --disable-manual --disable-libcurl-option

This effectively enables only HTTP and FTP protocol support.
Remove unnecessary NEED_VORBIS since you have USE_CODEC_VORBIS

Thanks zturtleman for catching this
Clear referenced paks when disconnecting

After playing Team Arena, if you switch to Q3A and try to play a demo with
cl_allowDownload enabled it would print "Need Paks: blah blah" and not play the demo.
Merge branch 'master' into features/upgrade-curl-7.35.0
Fix sending pk3 download error messages to clients

Disabled in e52a492f615e1245d6d22a84d2cc100c6d2f9f88.
Add support for x86_64 cross compiling in Travis CI
Merge pull request #45 from xhairball/features/upgrade-curl-7.35.0

Upgrade libcurl to the latest release (7.35.0).
OpenGL2: Set RGBM to use a multiplier of 1, and only use it with HDR lightmaps.
OpenGL2: Fix bug in ColorToRGBM().
OpenGL2: Oops, set lightmap alpha to 1.
Rebuilt Windows libcurl-7.35.0 static libraries

Fix linking with mingw32.
Enable FTPS and HTTPS.