Adjust CPP_FOR_BUILD

Hi.

CPP/CPPFLAGS were changed by commit 84401ce5.  That commit uses CPP as a default for CPP_FOR_BUILD.  Unless CPP is defined, GNU make defaults CPP as `$(CC) -E'.  Given the context, this is now incorrect, since CC_FOR_BUILD should be used.

Fixes PR103011.

-- Pekka

gcc/Changelog:

	* configure: Regenerate.
	* configure.ac: For CPP_FOR_BUILD use $(CC_FOR_BUILD) -E instead of
	$(CPP).