[CMake] Don't enable BUILD_WITH_INSTALL_RPATH when using custom build rpath

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.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D93177