Do not install docs when gtk-doc is not installed

Reported-by: Dagobert Michelsen <dam@opencsw.org>

Makefile.am

@@ -1,10 +1,11 @@
 # got some hints from https://gitorious.org/openismus-playground/examplelib/source
 
-SUBDIRS = po include src tools data docs/libpsl tests
+SUBDIRS = po include src tools data $(LIBPSL_DOCS) tests
+
 ACLOCAL_AMFLAGS = -I m4 ${ACLOCAL_FLAGS}
 
 # Enable GTK-Doc during make distcheck
-DISTCHECK_CONFIGURE_FLAGS = --enable-gtk-doc --enable-man
+#DISTCHECK_CONFIGURE_FLAGS = --enable-gtk-doc --enable-man
 
 ## Install the generated pkg-config file (.pc) into the expected location for
 ## architecture-dependent package configuration information. Occasionally,

configure.ac

@@ -36,6 +36,12 @@ m4_ifdef([GTK_DOC_USE_LIBTOOL], [], [
 AM_CONDITIONAL([GTK_DOC_USE_LIBTOOL], false)
 ])
 
+if test x"$have_gtk_doc" = xyes -a x"$enable_gtk_doc" = xyes; then
+  docdir_makefile=docs/libpsl/Makefile
+  docdir_version_xml=docs/libpsl/version.xml
+  AC_SUBST([LIBPSL_DOCS], [docs/libpsl])
+fi
+
 #
 # enable creation of man pages
 #
@@ -247,8 +253,8 @@ AC_CONFIG_FILES([Makefile
                  src/Makefile
                  tools/Makefile
                  po/Makefile.in
-                 docs/libpsl/Makefile
-                 docs/libpsl/version.xml
+                 ${docdir_makefile}
+                 ${docdir_version_xml}
                  data/Makefile
                  tests/Makefile
                  libpsl.pc:libpsl.pc.in])