build: Add --disable-documentation option

For embedded systems, it's pointless to build the docs if they're not
going to be used/shown.

Also, for gnome-ostree right now I don't have the Docbook
infrastructure set up.

Makefile.am

@@ -1,6 +1,9 @@
 ACLOCAL_AMFLAGS = -I m4 ${ACLOCAL_FLAGS}
 
-SUBDIRS = po shell panels man
+SUBDIRS = po shell panels
+if BUILD_DOCUMENTATION
+SUBDIRS += man
+endif
 DIST_SUBDIRS = po shell panels man
 
 MAINTAINERCLEANFILES = \

configure.ac

@@ -35,7 +35,17 @@ x_libs="$X_PRE_LIBS $X_LIBS -lX11 $X_EXTRA_LIBS"
 
 AC_PATH_PROG([GLIB_MKENUMS],[glib-mkenums])
 
+AC_ARG_ENABLE(documentation,
+              AC_HELP_STRING([--enable-documentation],
+                             [build documentation]),,
+              enable_documentation=yes)
+if test x$enable_documentation = xyes; then
    AC_PATH_PROG([XSLTPROC], [xsltproc])
+   if test x$XSLTPROC = x; then
+      AC_MSG_ERROR([xsltproc is required to build documentation])
+   fi
+fi
+AM_CONDITIONAL(BUILD_DOCUMENTATION, test x$enable_documentation = xyes)
 
 dnl Region panel
 savecppflags=$CPPFLAGS