CONFIGURE: Remove POSIXisms in configure.

This allows configure to work fine on Solaris 10 again. This fixes bug
 #3614513 "A few posixisms have crept into configure (w/ patch)".

Thanks to lblume for his patch.
SDL: Fix compilation on Solaris 10.

This replaces a dummy FILE definition before including the SDL headers with
simply using the toolchain's definition on Solaris. This is pretty harmless
because we only allow FILE to be used in the SDL headers by this.

Fixes bug #3614514 "#define FILE FAKE_FILE doesn't work on Solaris (w/ patch)".

Thanks to lblume for his patch!