Cleanup autoconf compiler support

Cleanup autoconf compiler support

Get rid of some old useless compiler checking code

Do not touch CXXFLAGS and friends, only the AM_* version

Let the gcc-related options for latest known version be the default (assume they will still be fine in future versions)

Try to take clang in account in a better way (to be continued):
 * use proper option for C++11,
 * use -Wno-deprecated-register in this case because Qt has a lot of these
 * define version as being clang for internal testing (there is no real way to get the clang version)