The testcase gfortran.dg/PR100914.f90 that I recently checked in 
(originally written by José Rui Faustino de Sousa) depends on the 
<quadmath.h> header file to obtain a typedef for __complex128.  It 
appears not to be possible to define an equivalent type in a portable 
way in the testcase itself (see 
https://gcc.gnu.org/onlinedocs/gcc-11.2.0/gcc/Floating-Types.html) so 
this patch skips the test entirely on targets where quadmath.h is not 
available.

The target-supports.exp change was cut-and-pasted from similar code in 
that file, but I haven't figured out how to test this change in a build 
that doesn't provide quadmath.h (e.g., my aarch64-linux-gnu toolchain 
build attempt croaked with an unrelated compilation error in glibc). 
Perhaps someone who previously encountered the FAILs on this testcase 
can confirm that it's skipped with this change?

-Sandra