Put doc in /usr/share/doc/texmf/ instead of /usr/share/texmf/doc

On Debian systems, the second is a symlink on the former.
--- a/src/Makefile
+++ b/src/Makefile
@@ -16,7 +16,7 @@
 
 LATEXDIR=$(texmf_prefix)/tex/latex/$(PKGNAME)
 LATEXCFGDIR=$(texmf_prefix)/tex/latex/$(PKGNAME)
-DOCDIR=$(texmf_prefix)/doc/latex/$(PKGNAME)
+DOCDIR=$(prefix)/share/doc/texmf/latex/$(PKGNAME)
 SCRIPTSDIR=$(texmf_prefix)/scripts/$(PKGNAME)
 
 SCRIPTS=src/figdepth.pl src/gensubfig.sh src/latexfilter.pl