Tests: Avoid OS X 10.5 limitation warning in RunCMake.install test

The EXPORT-OldIFace test case uses install(TARGETS) and so generates a
warning:

 CMake Warning in CMakeLists.txt:
   WARNING: Target "foo" has runtime paths which cannot be changed during
   install.  To change runtime paths, OS X version 10.6 or newer is required.
   Therefore, runtime paths will not be changed when installing.
   CMAKE_BUILD_WITH_INSTALL_RPATH may be used to work around this limitation.

Set CMAKE_BUILD_WITH_INSTALL_RPATH to avoid the warning since we do not
need to run the binaries from the build tree anyway.

Tests/RunCMake/install/EXPORT-OldIFace.cmake

 enable_language(C)
+set(CMAKE_BUILD_WITH_INSTALL_RPATH 1)
 add_subdirectory(EXPORT-OldIFace)
 add_library(foo SHARED empty.c)
 target_link_libraries(foo bar)