When BUILD_WITH_INSTALL_RPATH is enabled it prevents using a custom rpath only
for the build tree as the install rpath will be used. This makes it impossible to run a
runtimes build when compiling with Clang and wanting the installed rpath to be
empty (i.e. -DCMAKE_BUILD_RPATH="<some path>" -DCMAKE_SKIP_INSTALL_RPATH=ON).
Disable BUILD_WITH_INSTALL_RPATH when CMAKE_BUILD_RPATH is non-empty to
allow for such build scenarios.