Various fixes :
a) Don't abort configuration when no SVN working directory.
b) Don't overwrite CMAKE_INSTALL_PREFIX when a value is already assigned.
   Usage :
   cmake -DCMAKE_INSTALL_PREFIX=$oorexx_delivery $oorexx_src_dir
c) Fix INSTALL_LIB_DIR : the detection of lib64 does not work.
Fix compilation error with clang
Use "/tmp/ooRexx.pid" instead of "/var/run/ooRexx.pid"
(tired to get ooRexx crash because root access needed to launch rxapi)
Use a RO access to SVN repository, to avoid accidental update