build: allow linux and osx to build against static qt5

This is the first part of a huge effort to rework the handling of dependencies.
To start, this change allows all supported platforms to build against a static
Qt. 5.2.1 and 5.3 have been successfully tested against osx64, win32, win64,
linux32, and linux64.

It also makes a small change to the windows config, to allow linking against
qt builds with or without built-in libjpeg/libpng/libpcre/libz.

The actual build processes to take advantage of these changes (for gitian and
pull-tester) are coming soon. Until then, this should be a no-op.

configure.ac

@@ -258,6 +258,9 @@ case $host in
 
      CPPFLAGS="$CPPFLAGS -DMAC_OSX"
      ;;
+   *linux*)
+     TARGET_OS=linux
+     ;;
    *)
      ;;
 esac