build-sys: make gtk-doc dependency optional

Makefile.am

@@ -1276,8 +1276,10 @@ EXTRA_DIST += \
 	src/libsystemd-daemon/libsystemd-daemon.sym
 
 # ------------------------------------------------------------------------------
+if ENABLE_GTK_DOC
 SUBDIRS += \
 	docs/libudev
+endif
 
 include_HEADERS += \
 	src/libudev/libudev.h
@@ -1625,8 +1627,10 @@ dist_udevrules_DATA += \
 
 # ------------------------------------------------------------------------------
 if ENABLE_GUDEV
+if ENABLE_GTK_DOC
 SUBDIRS += \
 	docs/gudev
+endif
 
 libgudev_includedir = \
 	$(includedir)/gudev-1.0/gudev

autogen.sh

@@ -21,7 +21,17 @@ if [ -f .git/hooks/pre-commit.sample -a ! -f .git/hooks/pre-commit ] ; then
     echo "Activated pre-commit hook."
 fi
 
-gtkdocize
+GTKDOCIZE=`which gtkdocize`
+if test -z $GTKDOCIZE; then
+    echo "You don't have gtk-doc installed, and thus"
+    echo "won't be able to generate the documentation."
+    NOGTKDOC=1
+    echo 'EXTRA_DIST =' > gtk-doc.make
+fi
+
+if test -z "$NOGTKDOC"; then
+    gtkdocize || exit $?
+fi
 intltoolize --force --automake
 autoreconf --force --install --symlink
 

configure.ac

@@ -61,7 +61,16 @@ AM_PROG_CC_C_O
 AC_PROG_GCC_TRADITIONAL
 
 AC_PATH_PROG([M4], [m4])
-GTK_DOC_CHECK(1.10)
+
+# gtkdocize greps for ^GTK_DOC_CHECK and parses it, so you need to have
+# it on it's own line.
+m4_ifdef([GTK_DOC_CHECK], [
+GTK_DOC_CHECK([1.18])
+],[
+AM_CONDITIONAL([ENABLE_GTK_DOC],[false])
+])
+
+
 GOBJECT_INTROSPECTION_CHECK([1.31.1])
 
 AC_CHECK_TOOL(OBJCOPY, objcopy)