Linux: default to clang build
Linux: default to clang build

The build will now fail if clang is not found. To force the use
of GCC, use:

   make USE_GCC=1

The toolchain makefile was modified so it no longer checks for
various versions of clang if USE_GCC=1 is passed.

Signed-off-by: Mark Charlebois <f1891e7fc976ae1bd489627e2954fb730751d728@gmail.com>

mje-nz/PX4-Firmware,mje-nz/PX4-Firmware,darknight-007/Firmware,krbeverx/Firmware,acfloria/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,mje-nz/PX4-Firmware,PX4/Firmware,acfloria/Firmware,mcgill-robotics/Firmware,acfloria/Firmware,jlecoeur/Firmware,PX4/Firmware,krbeverx/Firmware,PX4/Firmware,jlecoeur/Firmware,mje-nz/PX4-Firmware,mcgill-robotics/Firmware,darknight-007/Firmware,jlecoeur/Firmware,dagar/Firmware,dagar/Firmware,Aerotenna/Firmware,mcgill-robotics/Firmware,mcgill-robotics/Firmware,mcgill-robotics/Firmware,Aerotenna/Firmware,dagar/Firmware,jlecoeur/Firmware,Aerotenna/Firmware,PX4/Firmware,jlecoeur/Firmware,mje-nz/PX4-Firmware,PX4/Firmware,krbeverx/Firmware,acfloria/Firmware,krbeverx/Firmware,PX4/Firmware,jlecoeur/Firmware,dagar/Firmware,Aerotenna/Firmware,darknight-007/Firmware,jlecoeur/Firmware,mcgill-robotics/Firmware,jlecoeur/Firmware,acfloria/Firmware,dagar/Firmware,darknight-007/Firmware,Aerotenna/Firmware,krbeverx/Firmware,Aerotenna/Firmware,dagar/Firmware,PX4/Firmware,acfloria/Firmware,krbeverx/Firmware,Aerotenna/Firmware,mcgill-robotics/Firmware,acfloria/Firmware,krbeverx/Firmware,mje-nz/PX4-Firmware,darknight-007/Firmware