Skip LATEX_USABLE guard as this done in dune_add_latex_document.

Otherwise the pdfs will not be installed from the tarballs.

[[Imported from SVN: r1888]]

doc/CMakeLists.txt

 add_subdirectory("doxygen")
-if(LATEX_USABLE)
-  dune_add_latex_document(istl.tex FATHER_TARGET doc
-    BIB_FILES istl.bib  DEFAULT_SAFEPDF IMAGES blockstructure.eps)
-  create_doc_install(istl.pdf
-    ${CMAKE_INSTALL_DOCDIR} istl_safepdf)
-endif(LATEX_USABLE)
+dune_add_latex_document(istl.tex FATHER_TARGET doc
+  BIB_FILES istl.bib  DEFAULT_SAFEPDF IMAGES blockstructure.eps)
+create_doc_install(istl.pdf
+  ${CMAKE_INSTALL_DOCDIR} istl_safepdf)