texturepacker: crosscompile fixes

this fixes:
- libsquish-native.so will be built even if TextuePacker is disabled
- libsquish-native.so will be built for target
- native TextuePacker will not be built if TexturePacker is enabled and we cross-compile

This change:
- adds '--with-texturepacker-root=" to configure script to configure the location of the host library and include dir to search if native T
exturePacker should be build. If not configured it falls back to $prefix (the default location for most host tools). This location can be still hardcoded in configure.in with 'use_texturepacker_root='.
- if we enable TexturePacker building and we crosscompile we force the native building of TexturePacker.
- using $HOST_CXX for building native TexturePacker and native libsquish and fall back to 'g++'.
- using $HOST_CXXFLAGS and $HOST_LIBS for building native TexturePacker and native libsquish if configured.