a2x: adding a check for needed helper

The previous commit added some asciidoc based man pages that need a2x
from the asciidoc package. To ensure the build will not break due a not
available a2x binary we better check before.

configure.ac

@@ -236,6 +236,11 @@ AC_ARG_ENABLE([examples],
               [build_examples="yes"])
 
 if test "x$build_examples" = "xyes"; then
+	AC_PATH_PROG([A2X], [a2x])
+	if test "x$A2X" = "x"; then
+		AC_MSG_WARN([==> You want to build the examples binaries, that is also including the respective manpages but a2x was not found!])
+		AC_MSG_ERROR([==> Install the package that contains the a2x binary or disable the build of the examples using --disable-examples.])
+	fi
 	BUILD_EXAMPLES="YES"
 fi
 AM_CONDITIONAL(BUILD_EXAMPLES, [test "x$build_examples" != "x"])
@@ -372,4 +377,5 @@ else
 fi
 if test "x$build_examples" = "xyes"; then
 	AC_MSG_RESULT([      build examples          : "yes"])
+	AC_MSG_RESULT([             -->  a2x around  : "yes" ($A2X)])
 fi

examples/Makefile.am

@@ -33,7 +33,7 @@ MAN5 = $(TXT5:%.txt=%.5)
 man5_MANS = $(MAN5)
 
 .txt.5:
-	a2x --doctype manpage --format manpage $<
+	$(A2X) --doctype manpage --format manpage $<
 
 CLEANFILES = *.5