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).