cmake: Fix incorrect relative-path concatenation.

When a `-DCMAKE_INSTALL_PREFIX` is given, this could result in paths
like:

    /path/to/prefix//path/to/prefix/lib/cmake/glog/glog-modules.cmake

which was incorrect.

If a relative DESTINATION is given, it's assumed to be relative to the
install prefix.

Signed-off-by: Niklas Hambüchen <mail@nh2.me>

CMakeLists.txt

@@ -868,7 +868,7 @@ configure_file (\"${CMAKE_CURRENT_SOURCE_DIR}/glog-modules.cmake.in\"
 file (INSTALL
   \"${CMAKE_CURRENT_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeTmp/glog-modules.cmake\"
   DESTINATION
-  \"\${CMAKE_INSTALL_PREFIX}/${_glog_CMake_INSTALLDIR}\")
+  \"${_glog_CMake_INSTALLDIR}\")
 "
   COMPONENT Development
 )