build: allow correct tools to be used for dmg creation.

These come from the enironment, which will be properly setup by Make with
the paths gleaned from configure.
Also don't crash if plugins are static.
build: fix qt.conf case-sensitivity in the deployed dmg

For qt5.2 on osx, the qcocoa plugin is mandatory. However, it fails to load
when qt.conf specifies the "plugin" path instead of the expected "Plugin". This
is in line with the documentation:
https://qt-project.org/doc/qt-5.0/qtdoc/qt-conf.html

I'm not sure how the plugins were loading before, unless the case-sensitivity
for OSX is new.
build: if cross-compiling for an apple host, locate some additional tools

This should be safe to do for native builds too, but for now it's
specific to cross-builds to avoid possible regressions.
build: ensure the correct strip is used for osx/win32
build: hook up "make deploy" for cross osx builds