Allow overriding target compiler and linker with qmake's one

Using make_global_settings, we can pass along the compiler and linker
that qmake uses, still allowing overriding it through environment variables.
We generate qmake_extras.gypi and populated with the host and target compilers
so that it also works when cross compiling.

Based on patch by Pierre Rossi <pierre.rossi@digia.com>

Change-Id: Ia3789abeea88f4d3cde75c2cf07cc9de66d7afbf
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>