Current autoconf versions are very unhappy when code fragments are not
wrapped in AC_LANG_SOURCE macros, generating errors like:

configure.ac:723: warning: AC_LANG_CONFTEST: no AC_LANG_SOURCE call detected in body
../../lib/autoconf/lang.m4:194: AC_LANG_CONFTEST is expanded from...
../../lib/autoconf/general.m4:2662: _AC_LINK_IFELSE is expanded from...
../../lib/autoconf/general.m4:2679: AC_LINK_IFELSE is expanded from...
../../lib/m4sugar/m4sh.m4:606: AS_IF is expanded from...
../../lib/autoconf/general.m4:2032: AC_CACHE_VAL is expanded from...
../../lib/autoconf/general.m4:2053: AC_CACHE_CHECK is expanded from...
configure.ac:723: the top level

Signed-off-by: Keith Packard <keithp@keithp.com>