try_compile: Trim whitespace from LINK_LIBRARIES entries

Commit e65ef08b (try_compile: Fix quoting of libraries in generated
CMakeLists.txt, 2013-05-24) added quoting to entries specified in
the LINK_LIBRARIES. However, if the input entries contain whitespace
padding, that quoted whitespace causes an error in the generated
CMakeLists.txt at target_link_libraries.

Strictly, it is an error to have space separated entries in the
CMAKE_REQUIRED_LIBRARIES, as it was never properly handled by
CMakeExpandImportedTargets even prior to commit 236133e7 (Handle
targets in the LINK_LIBRARIES of try_compile., 2013-02-09). However,
it is causing a regression in KDE code which tests the next branch.
Merge topic 'fix-try_compile-library-spaces' into next

e537544 try_compile: Trim whitespace from LINK_LIBRARIES entries