Try to fix Travis CI MinGW builds

Listing i686-w64-mingw32-gcc as a compiler stopped working.
Travis tries to print version before installing it.

$ i686-w64-mingw32-gcc --version

/home/travis/build.sh: line 201: i686-w64-mingw32-gcc: command not found

The command "i686-w64-mingw32-gcc --version" failed and exited with 127 during setup.

.travis.yml

@@ -1,9 +1,15 @@
 language: c
-compiler:
-  - gcc
-  - clang
-  - i686-w64-mingw32-gcc
-  - x86_64-w64-mingw32-gcc
+
+env:
+  # standard builds
+  - CC=gcc
+  - CC=clang
+  # extra libs
+  - CC=gcc USE_CODEC_VORBIS=1 USE_FREETYPE=1
+  - CC=clang USE_CODEC_VORBIS=1 USE_FREETYPE=1
+  # cross-compile using mingw
+  - CC= PLATFORM="mingw32" ARCH="x86"
+  - CC= PLATFORM="mingw32" ARCH="x86_64"
 
 script: ./travis-ci-build.sh