configure: add initial support for CC_FOR_BUILD, CXX_FOR_BUILD, CFLAGS_FOR_BUILD, CXXFLAGS_FOR_BUILD, LDFLAGS_FOR_BUILD
configure/depends: rename 'USE_TEXTUREPACKER_NATIVE_ROOT' to 'NATIVE_ROOT' for general usage to use with other hostbuilt tools. Add $NATIVE_ROOT/lib to LDFLAGS_FOR_BUILD and $NATIVE_ROOT/include to CFLAGS_FOR_BUILD and CXXFLAGS_FOR_BUILD
libsquish: rework to support native builds with *_FOR_BUILD
Texturepacker: rework to support native builds with *_FOR_BUILD, Texturepacker builds always against libsquish-native.so, which is ok because CXX_FOR_BUILD=CXX, CXXFLAGS_FOR_BUILD=CXXFLAGS and LDFLAGS_FOR_BUILD=LDFLAGS on non crosscompiled builds, sothere libsquish-native is the same like libsquish
configure: cleanup, we dont need 'use_texturepacker_native' anymore and dont need to force 'use_texturepacker' (its enabled by default).
Merge pull request #3190 from sraue/texturepacker-rework

Texturepacker rework