meson: cleanup win32 library detection

meson: cleanup win32 library detection

As pointed out by Akihiko Odaki, all Win32 libraries in MinGW have lowercase
names.  This means that on (case-insensitive) Windows you can use the mixed-case
names suggested by Microsoft or all-lowercase names, while on Linux you need to
make them lowercase.

QEMU was already using lowercase names, so there is no need to test the
mixed-case name version of libSynchronization.  Remove the unnecessary test
and while at it make all the tests use "required: true".

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>