meson: accept either shared or static libraries if --disable-static

meson: accept either shared or static libraries if --disable-static

Meson's "static" argument to cc.find_library is a tri-state.  By default
Meson *prefers* a shared library, which basically means using -l to
look for it; instead, "static: false" *requires* a shared library.  Of
course, "static: true" requires a static library, which is all good
for --enable-static builds.

For --disable-static, "static: false" is rarely desirable; it does not
match what the configure script used to do and the test is more complex
(and harder to debug if it fails, which was reported by Peter Lieven
for librbd).

Reported-by: Peter Lieven <pl@kamp.de>
Tested-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>