Add CMake 2 support

[tresf]

Partially reverts d599955 and fixes CMake 2 support when building with MingGW. Originally reported by @Umcaruje on #programming channel on Discord, CMake 3 git blame discovered by @PhysSong.

This patch does not need to be fast-forwarded to master branch because master requires CMake 3 already although it should be at least hand-patched because d599955 was accidentally reverted at some point.

Ubuntu 14.04 still defaults to CMake 2 by default (e.g. apt-get install cmake), so this can help avoid unobvious dependency issues.

Before:

-- Found PkgConfig: /usr/bin/pkg-config (found version "0.26")

After:

-- Found PkgConfig: /opt/mingw64/bin/x86_64-w64-mingw32-pkg-config (found version "0.26")

Keywords:

-- checking for module 'sndfile>=1.0.11'
--   package 'sndfile>=1.0.11' not found
CMake Error at /usr/share/cmake-2.8/Modules/FindPkgConfig.cmake:283 (message):
  A required package was not found
Call Stack (most recent call first):
  /usr/share/cmake-2.8/Modules/FindPkgConfig.cmake:337 (_pkg_check_modules_internal)
  CMakeLists.txt:189 (PKG_CHECK_MODULES)


CMake Error at CMakeLists.txt:191 (MESSAGE):
  LMMS requires libsndfile1 and libsndfile1-dev >= 1.0.11 - please install,
  remove CMakeCache.txt and try again!

[tresf]

Note, until this PR is merged, the documentation temporarily states that cmake3 is required. That note should be removed once merged. Edit: Removed cmake3 note from documentation.