Win32 build is broken, needs to find PathIsRelativeA()

Created by: nicowilliams

linker.c needs to include <windows.h> and <shlwapi.h>, and jq needs to link with -lshlwapi when building with mingw. I'm not sure if the latter is something we should do unconditionally in configure.ac, so I'll not fix that (instead we can point out the need to add it to LDFLAGS in the ./configure incantation).