Fix libbacktrace -fPIC breakage from "Use libbacktrace in libgfortran"

[FX]

> PIC_FLAG=
> if test -n "${with_target_subdir}"; then
> -  PIC_FLAG=-fPIC
> +  AC_TRY_COMPILE([void foo(void){}], [PIC_FLAG=-fPIC])
> fi

There’s something I don’t understand about this test. Shouldn’t AC_TRY_COMPILE take a first (empty) argument, “includes”? And shouldn’t you first set the -fPIC flag before try to compile with that?

FX