qt: fix build with zlib for target

This contains a few hacks very specific to Qt's buildsystem. These can be
reverted once we split the build between native and target builds.

Qt's build contains a circular dependency when not using a system zlib.
By far the easiest fix is to switch to a system zlib, rather than Qt's own.
However, that confuses Qt's cross build which assumes that when using a system
zlib, it should also find a system (native) zlib for native tools. The build
breaks if that zlib is not present.

To solve this:
1. Always use a system zlib rather than the one provided by qt
2. Set force_bootstrap, which instructs the build tools to be built as though
   we're cross-compiling (build != target)
3. For build tools, use qt's internal zlib so that a native zlib is not
required.

Step 3 means that if any zlib headers are found by the native build, it will
confuse Qt's internal zlib build. So we also need to make sure that the target
headers/libs aren't found. To do so, specify that our
cflags/cxxflags/cppflags/ldflags only apply for non-host builds.

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