We need different defines/cflags for building static and shared builds of
libsigc++, so we can't really support default_library = 'both' for libsigc++
without much retinkering.

So, just disallow such builds at least for now.

Also, save up whether we are attempting a static build in the Visual Studio
build.

...and add a new check macro LIBSIGCXX_STATIC, to use the appropriate macros to
build and link against libsigc++. Drop this from the build files as well.

Apply -DLIBSIGCXX_STATIC as appropriate when we request a static build to be
done for the Meson and NMake builds, and skip building the version .rc file
if a static build is requested.

For the NMake builds, separate the build artifacts from the static and shared
builds.

The autotools builds since it is not used for Visual Studio builds at all.

Copy the built DLL and PDB only if building a shared build, and copy the
appropriate .lib file according to the build type.

Update the Meson build files to put in -DLIBSIGCXX_STATIC when we are building
a static build of libsigc++.

For the autotools build, this is not used.