CGAL always builds dynamic

[vector76]

I found that when attempting to build cgal with static linkage, it instead built dynamic, i.e. libCGAL.dll.a, libCGAL_Core.dll.a, libCGAL_ImageIO.dll.a, and libCGAL_Qt4.dll.a.

I believe that part of the most recent change to cgal.mk, adding
-DCGAL_BUILD_SHARED_LIBS=$(CMAKE_SHARED_BOOL)
in place of
-DBUILD_SHARED_LIBS=OFF
is incorrect.

According to CGAL manual http://doc.cgal.org/latest/Manual/installation.html#title13

Shared libraries, also called dynamic-link libraries, are built by default (.dll on Windows, .so on Linux, .dylib on MacOS). You can choose to produce static libraries instead by setting the CMake variable BUILD_SHARED_LIBS to FALSE.

So I suspect that -DCGAL_BUILD_SHARED_LIBS=ON/OFF has no effect and the shared-by-default is taking effect.

For my purpose I hard-coded static by modifying cgal.mk, but of course this is not a solution that will support both static and dynamic.

If I can figure out a solution that properly respects CMAKE_SHARED_BOOL and if I can figure out github forking and pull requests, I'll try to submit a pull request, but both are new to me.