pkg-cmake.mk: build shared library when !BR2_PREFER_STATIC_LIB

CMake offers a couple of places where one can specify how to build a
library:
- when you create the library target itself, by calling
  add_library(target [SHARED|STATIC] ...)
- or globally, when you configure the build, by setting the
  BUILD_SHARED_LIBS CMake flag.

* if the library target kind of library is specified:
    it overrides the global setting BUILD_SHARED_LIBS;
* else, if the global setting BUILD_SHARED_LIBS is set:
    it builds according to the BUILD_SHARED_LIBS flags;
* otherwise:
    for linux, it will build static library (like BUILD_SHARED_LIBS
    default is OFF).

So, we can consider the setting BUILD_SHARED_LIBS acts a bit similarly
to the autotools ones '--disable-static' and '--enable-shared'.
Thus, it makes sense for Buildroot to globally drive to CMake flags in
the cmake-package infrastructure.

It seems we never trigger this so far because:
- either we specified it in the *.mk file (e.g. opencv.mk);
- or it was already set per target by the projects' upstreams.

Followup patches will clean the remaining BUILD_SHARED_LIBS in the
package *.mk files.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>