CMake: Similar to 3fcf0968, use gdlib.pc if available

CMake: Similar to 3fcf0968, use gdlib.pc if available

The gdlib-config program is no longer installed with the newer versions
of the library as it is deprecated. The CMake build on CI for Fedora
prints the following, which is no longer the case with this patch:

   CMake Warning at cmake/FindGD.cmake:63 (message):
     gdlib-config not found; skipping feature checks
   Call Stack (most recent call first):
     contrib/diffimg/CMakeLists.txt:1 (find_package)

xref: https://github.com/libgd/libgd/issues/376#issuecomment-276138975

Edits from Matthew Fernandez:
 - Wrap CMake line
 - Adjust #1786 work arounds

Gitlab: #1786