[poppler] Fix building static libraries with cmake

[Jeroen Ooms]

Several projects use static builds of poppler-cpp to ship standalone
pdf applications, but since the switch to cmake it is no longer
possible to build static libs.

Setting -DBUILD_SHARED_LIBS=OFF in cmake only builds a static
libpoppler.a, however libpoppler-cpp still gets built as a dynamic
library (because SHARED is hardcoded in CMakeLists.txt).

It would be really great of cmake could build both the static and the
shared libraries, like many other C/C++ libs do. The patch below
implements this:

  Human readable view: https://github.com/tsdgeos/poppler_mirror/pull/2/files
  Patch file: https://github.com/tsdgeos/poppler_mirror/pull/2.patch

What can I do to get this patch reviewed?