[libsigcplusplus] configure.ac: Remove check for m4.

[Murray Cumming <murrayc src gnome org>]

commit 9c47c90187a8e64dc379ae28f928646796efc2e2
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Mar 18 09:52:01 2016 +0100

    configure.ac: Remove check for m4.
    
    As suggested by Kjell:  https://bugzilla.gnome.org/show_bug.cgi?id=763515#c2

 configure.ac |    6 ------
 1 files changed, 0 insertions(+), 6 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 91808ad..9525931 100644
--- a/configure.ac
+++ b/configure.ac
@@ -40,12 +40,6 @@ MM_AX_CXX_COMPILE_STDCXX([14],[noext],[mandatory])
 AC_DISABLE_STATIC
 LT_INIT([win32-dll])
 
-AC_ARG_VAR([M4], [M4 macro processor command])
-AC_CHECK_PROGS([M4], [gm4 m4])
-
-AS_IF([test "x$M4" = x],
-      [AC_MSG_FAILURE([[The GNU M4 macro processor is required for building libsigc++.]])])
-
 MM_ARG_ENABLE_DOCUMENTATION
 MM_ARG_WITH_TAGFILE_DOC([libstdc++.tag], [mm-common-libstdc++])