[easytag] Document that Yelp is required at runtime

[David King <davidk src gnome org>]

commit 4df33c1f7b719ea59d9a054b009ec2217b261b8b
Author: David King <amigadave amigadave com>
Date:   Sun Feb 28 21:35:02 2016 +0000

    Document that Yelp is required at runtime
    
    Warn during configure if yelp is not found in the path.
    
    https://mail.gnome.org/archives/easytag-list/2016-February/msg00009.html

 README       |    1 +
 configure.ac |    4 ++++
 2 files changed, 5 insertions(+), 0 deletions(-)
---
diff --git a/README b/README
index 4a2bcf0..7b4388d 100644
--- a/README
+++ b/README
@@ -67,6 +67,7 @@ Installation
 * opus and opusfile (http://www.opus-codec.org/) (if not deactivated by './configure --disable-opus')
 * taglib (http://taglib.github.com/) (if not deactivated by './configure --disable-mp4')
 * wavpack (http://www.wavpack.com/) (if not deactivated by './configure --disable-wavpack')
+* yelp (https://wiki.gnome.org/Apps/Yelp) is required at runtime for showing the user help
 * yelp-tools (https://git.gnome.org/browse/yelp-tools)
 * libc 6 (glibc 2.1) or greater
 * appdata-tools (https://github.com/hughsie/appdata-tools)
diff --git a/configure.ac b/configure.ac
index de2b87f..d94ce2a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -114,6 +114,10 @@ dnl Help.
 m4_ifdef([YELP_HELP_INIT], [YELP_HELP_INIT],
          [AC_MSG_ERROR([yelp-tools is required to build EasyTAG help])])
 
+AC_PATH_PROG([YELP], [yelp])
+AS_IF([test "x$ac_cv_path_YELP" = "x"],
+      [AC_MSG_WARN([Yelp is required at runtime for user help])])
+
 dnl -------------------------------
 dnl Handle cross-compiling with mingw32.
 dnl -------------------------------