gnu: erlang: Fix man-pages search path of 'erl -man'.

* gnu/packages/patches/erlang-man-path.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/erlang.scm (erlang)[source]: Use it.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

gnu/local.mk

@@ -579,6 +579,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/emacs-fix-scheme-indent-function.patch	\
   %D%/packages/patches/emacs-scheme-complete-scheme-r5rs-info.patch	\
   %D%/packages/patches/emacs-source-date-epoch.patch		\
+  %D%/packages/patches/erlang-man-path.patch			\
   %D%/packages/patches/eudev-rules-directory.patch		\
   %D%/packages/patches/eudev-conflicting-declaration.patch	\
   %D%/packages/patches/evilwm-lost-focus-bug.patch		\

gnu/packages/erlang.scm

@@ -23,6 +23,7 @@
   #:use-module (guix build-system gnu)
   #:use-module (guix download)
   #:use-module (guix packages)
+  #:use-module (gnu packages)
   #:use-module (gnu packages autotools)
   #:use-module (gnu packages fontutils)
   #:use-module (gnu packages gl)
@@ -45,7 +46,8 @@
               (file-name (string-append name "-" version ".tar.gz"))
               (sha256
                (base32
-                "1b47jh549yywyp8fbs8a8j4ydr3zn982navzyqvlms6rg8vwb0pw"))))
+                "1b47jh549yywyp8fbs8a8j4ydr3zn982navzyqvlms6rg8vwb0pw"))
+              (patches (search-patches "erlang-man-path.patch"))))
     (build-system gnu-build-system)
     (native-inputs
      `(("perl" ,perl)

gnu/packages/patches/erlang-man-path.patch

@@ -0,0 +1,24 @@
+Patch originally from https://sources.debian.net/patches/erlang/1:20.0.1%2Bdfsg-2/man.patch/
+by Francois-Denis Gonthier <neumann@lostwebsite.net>.
+
+Patch description rewritten for Guix.
+
+This patch allows access to the man page with the 'erl -man' command
+(Erlang manual pages are placed to /gnu/store/..erlang../share/man/ hierarchy
+as other man pages.)
+
+--- a/erts/etc/common/erlexec.c
++++ b/erts/etc/common/erlexec.c
+@@ -709,8 +709,10 @@
+ 			error("-man not supported on Windows");
+ #else
+ 			argv[i] = "man";
+-			erts_snprintf(tmpStr, sizeof(tmpStr), "%s/man", rootdir);
+-			set_env("MANPATH", tmpStr);
++			/*
++			* Conform to erlang-manpages content.
++			*/
++			putenv(strsave("MANSECT=1:3:5:7"));
+ 			execvp("man", argv+i);
+ 			error("Could not execute the 'man' command.");
+ #endif