build-sys: fix AC_COMPILE_IFELSE tests

build-sys: fix AC_COMPILE_IFELSE tests

This removes some warnings when bootstrapping. The warnings were introduced
in autoconf 2.68. Reference:

http://www.flameeyes.eu/autotools-mythbuster/forwardporting/autoconf.html

The following code

	AC_COMPILE_IFELSE([
		<MY C CODE HERE>
		], ...)

is changed to

	AC_COMPILE_IFELSE(
		[AC_COMPILE_IFELSE([AC_LANG_SOURCE([
		<MY C CODE HERE>
		])], ...)