Texturepacker rework

This replaces https://github.com/xbmc/xbmc/pull/1090 and includes suggestions from @davilla and @theuni:

- adding initial support for *_FOR_BUILD variables for general usage. on normal builds CXX_FOR_BUILD=CXX, CXXFLAGS_FOR_BUILD=CXXFLAGS and LDFLAGS_FOR_BUILD=LDFLAGS and so on, on crosscompiling this variables can be specified or in case of CXX_FOR_BUILD and CC_FOR_BUILD it falls back to gcc/g++ defaults if avaible in path
- Rename  'USE_TEXTUREPACKER_NATIVE_ROOT' to 'NATIVE_ROOT' so it can be reused later for other hosttools
- porting libsquish and Texturepacker to use *_FOR_BUILD. libsquish-native will be built always and Texturepacker will link always to libsquish-native
- cleanup use_texturepacker_native and dont force use_texturepacker (enabled by default)

NATIVE_ROOT is the root location for hostlibraries and includes and is used to set include and library paths to *FLAGS_FOR_BUILD, but its fully optional if *FLAGS_FOR_BUILD is used to specify include and lib dirs.

Build tested on jenkins (OSX32, OSX64 and Android) and here on linux with crosscompiling and native builds.

Todo for later: i would recommend to build and link libsquish-native as static library, but i was unable todo this