Enable libstdc++ compilation in AVR targets with AVR-Libc. Most
floating point math functions are already defined in AVR-Libc, so
defines are in place to avoid multiple definition of these functions.

2016-12-06  Felipe Magno de Almeida  <felipe@expertisesolutions.com.br>

	* crossconfig.m4: Add avr target for cross-compilation.
	* configure: Regenerate.

From-SVN: r243309