Use $<INSTALL_PREFIX> in target_include_directories

To make sure packages are relocatable, use `$<INSTALL_PREFIX>` in
`$<INSTALL_INTERFACE:...>` `target_include_directories`.

`$<INSTALL_PREFIX>` was introduced in CMake 2.8.11, which is already
being checked for locally.

References:

- https://cmake.org/cmake/help/latest/manual/cmake-packages.7.html#id19
- https://github.com/robotology/how-to-export-cpp-library/blob/claudio/headeronly/src/LibTemplateCMake/CMakeLists.txt#L42

Signed-off-by: Dakota Hawkins <dakotahawkins@gmail.com>

googlemock/CMakeLists.txt

@@ -118,10 +118,10 @@ endif()
 if (DEFINED CMAKE_VERSION AND NOT "${CMAKE_VERSION}" VERSION_LESS "2.8.11")
   target_include_directories(gmock SYSTEM INTERFACE
     "$<BUILD_INTERFACE:${gmock_build_include_dirs}>"
-    $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)
+    "$<INSTALL_INTERFACE:$<INSTALL_PREFIX>/${CMAKE_INSTALL_INCLUDEDIR}>")
   target_include_directories(gmock_main SYSTEM INTERFACE
     "$<BUILD_INTERFACE:${gmock_build_include_dirs}>"
-    $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)
+    "$<INSTALL_INTERFACE:$<INSTALL_PREFIX>/${CMAKE_INSTALL_INCLUDEDIR}>")
 endif()
 
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