use fifth argument [action-if-cross-compiling] in AC_RUN_IF_ELSE

[Peter]

We use macro AC_RUN_IF_ELSE twice in configure.ac to test for quiet_NaN and infinity with a comment

# At run-time, but this has the disadvantage that yat cannot be built  
# for cross-compilation

However, there is a fifth argument available in the macro to be used when $cross_compiling=yes. I wonder if it wouldn't make sense to use this argument and fall back on compilation tests when cross compiling. For instance, for quiet_NaN there is already a compilation test checking if std::numeric_limits<double>::quiet_NaN() is available.

For more info see ​Autoconf Manual

[Peter]

fixed in [1358]