cmake: fix incorrect relative path concatenation (#733)

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.

AUTHORS

@@ -19,6 +19,7 @@ Guillaume Dumont <dumont.guillaume@gmail.com>
 Marco Wang <m.aesophor@gmail.com>
 Michael Tanner <michael@tannertaxpro.com>
 MiniLight <MiniLightAR@Gmail.com>
+Niklas Hambuechen <mail@nh2.me>
 romange <romange@users.noreply.github.com>
 Roman Perepelitsa <roman.perepelitsa@gmail.com>
 Sergiu Deitsch <sergiu.deitsch@gmail.com>

CMakeLists.txt

@@ -1027,7 +1027,7 @@ 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
 )

CONTRIBUTORS

@@ -39,6 +39,7 @@ Marco Wang <m.aesophor@gmail.com>
 Michael Darr <mdarr@matician.com>
 Michael Tanner <michael@tannertaxpro.com>
 MiniLight <MiniLightAR@Gmail.com>
+Niklas Hambuechen <mail@nh2.me>
 Peter Collingbourne <pcc@google.com>
 Rodrigo Queiro <rodrigoq@google.com>
 romange <romange@users.noreply.github.com>