AC_REQUIRE means "if this macro hasn't been executed already, execute
it". So in a wrapper around AC_RUN_IFELSE, AC_REQUIRE(AC_RUN_IFELSE)
isn't correct at that will execute AC_RUN_IFELSE without any arguments.
With autoconf 2.69 this is basically a no-op, but with autoconf 2.70,
AC_RUN_IFELSE without a default value when cross-compiling is fatal.
The result is that curl with autoconf 2.70 cannot cross-compile.
Fixes https://github.com/curl/curl/issues/5126
Closes https://github.com/curl/curl/pull/5130
f25f602ffd