make --with-gcc-toolchain optional.

The correct value will be extracted automatically from the --prefix
option as printed by the --with-cc specified gcc compiler.
Also add a check in case there are more than 10 include paths as in that
case the code in TCling.cxx and rootcling.cxx need to be extended.
when cxx11 is set on OS X, also turn the required libcxx11 option on (ROOT-5489).