Read in content of ROOT_INCLUDE_DIR and ROOT_GCC_TOOLCHAIN

Following the ideas of a nice patch proposed by David Abdurachmanov
and the current code in TCling, also rootcling is now able to
look for headers in the ROOT_INCLUDE_DIR env variable and to set
the toolchain to the content of ROOT_GCC_TOOLCHAIN.
The code has been refactored in TMetaUtils to avoid duplications.