There are better ways to do this, e.g. meson cmake subproject,
but that requires cmake 3.7 and some of our CI environments
only provide cmake 3.5.

Nor can we add a meson.build file to capstone/, because the git
submodule would then always report "untracked files".  Fixing that
would require creating our own branch on the qemu git mirror, at
which point we could just as easily create a native meson subproject.

Instead, build the library via the main meson.build.

This improves the current state of affairs in that we will re-link
the qemu executables against a changed libcapstone.a, which we wouldn't
do before-hand.  In addition, the use of the configuration header file
instead of command-line -DEFINES means that we will rebuild the
capstone objects with changes to meson.build.

Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>