https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109818
--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> --- This means djgpp is doing this: dnl To use the obsolete 'c_std' headers use --enable-cheaders-obsolete as dnl well as --enable-cheaders=c_std, otherwise configure will fail. Any idea why? As it says, the include/c_std/cmath header is obsolete, and is unmaintained and so has hardly any of the C99 <math.h> functions. The supported include/c_global/cmath header has trunc, and all the other math functions.
