changes in configure for libc++ support.

1) renamed option from --enable-libcxx11 to --enable-libcxx
2) automatically turn on --enable-libcxx when compiling with clang and C++11.
3) check for compatibility in external C++ libraries to be linked against
   libc++. Libraries linked with incompatible run-time are skipped (typically
   xrootd, pythia8 and oracle instant client libs).