Without the change build fails on upstream gcc-10 as:
ld: .libs/mach_dep.o:sigscheme/libgcroots/include/private/gc_priv.h:2029: multiple definition of
`GCROOTS_jmp_buf'; .libs/mark.o:sigscheme/libgcroots/include/private/gc_priv.h:2029: first defined here