Misc: Create directories for icons before installing them

Misc: Create directories for icons before installing them. This fixes
the install failure if the DESTDIR used when executing `make install`
does not contain the icon directories. It could appear when the user
does not wish to directly install the software to the system, like in
Gentoo ebuild or Arch Linux AUR building.

Makefile

@@ -156,6 +156,9 @@ install: $(BINS) docs
 ifneq "$(MANPAGES)" ""
 	@install -m644 $(MANPAGES) "$(DESTDIR)$(MANDIR)"/
 endif
+	@install -d \
+		"$(DESTDIR)$(ICODIR)/scalable/apps" \
+		"$(DESTDIR)$(ICODIR)/48x48/apps"
 	@install -m644 media/compton.svg "$(DESTDIR)$(ICODIR)/scalable/apps"/
 	@install -m644 media/icons/48x48/compton.png "$(DESTDIR)$(ICODIR)/48x48/apps"/
 	@install -m644 compton.desktop "$(DESTDIR)$(APPDIR)"/