Also, I think you should be able to fix this by editing 3rdparty/ff-petsc/Makefile and adding the flag -D_POSIX_C_SOURCE=199309L to COPTFLAGS there.
COPTFLAGS='-O3 $(FLAGS_MTUNE)' → COPTFLAGS='-D_POSIX_C_SOURCE=199309L -O3 $(FLAGS_MTUNE)'.
Then, simply make petsc-slepc again.