Use relative path for include/janet.h symlink

When using make to build an rpm, the current symlink is
created with an absolute path to the buildroot as causes
the make install target to fail with:

error: Symlink points to BuildRoot: /usr/include/janet.h -> /home/stephen/rpmbuild/BUILDROOT/janet-1.23.0-3.x86_64/usr/include/janet/janet.h

We can create the link relatively which makes this more
portable, where:

ln -sf -t '/home/stephen/rpmbuild/BUILDROOT/janet-1.23.0-3.x86_64/usr/include' janet.h janet/janet.h

Resulting in the following symlink:

ls -la BUILDROOT/usr/include/janet.h
lrwxrwxrwx. 1 stephen stephen 13 Jul  2 08:17 BUILDROOT/usr/include/janet.h -> janet/janet.h

This symlink can then be properly packaged without path
issues.

Signed-off-by: Stephen Hassard <steve@hassard.net>

Makefile

@@ -283,7 +283,7 @@ install: $(JANET_TARGET) $(JANET_LIBRARY) $(JANET_STATIC_LIBRARY) build/janet.pc
 	cp $(JANET_TARGET) '$(DESTDIR)$(BINDIR)/janet'
 	mkdir -p '$(DESTDIR)$(INCLUDEDIR)/janet'
 	cp -r build/janet.h '$(DESTDIR)$(INCLUDEDIR)/janet'
-	ln -sf '$(DESTDIR)$(INCLUDEDIR)/janet/janet.h' '$(DESTDIR)$(INCLUDEDIR)/janet.h'
+	ln -sf -t '$(DESTDIR)$(INCLUDEDIR)' janet.h janet/janet.h
 	mkdir -p '$(DESTDIR)$(JANET_PATH)'
 	mkdir -p '$(DESTDIR)$(LIBDIR)'
 	if test $(UNAME) = Darwin ; then \