Foundation for selecting plugins in configure

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

m4/.gitignore

@@ -1 +1,2 @@
 *.m4
+!plugin.m4

m4/plugin.m4

@@ -0,0 +1,43 @@
+# From https://github.com/collectd/collectd/blob/master/configure.ac
+# Dependency handling currently unused
+
+m4_define([my_toupper], [m4_translit([$1], m4_defn([m4_cr_letters]), m4_defn([m4_cr_LETTERS]))])
+
+# AC_PLUGIN(name, default, info)
+# ------------------------------------------------------------
+AC_DEFUN([AC_PLUGIN],[
+    m4_divert_once([HELP_ENABLE], [
+Optional Plugins:])
+    enable_plugin="no"
+    force="no"
+    AC_ARG_ENABLE([$1], AS_HELP_STRING([--enable-$1-plugin], [$3]), [
+    if test "x$enableval" = "xyes"; then
+             enable_plugin="yes"
+    else if test "x$enableval" = "xforce"; then
+             enable_plugin="yes"
+             force="yes"
+    else
+             enable_plugin="no (disabled on command line)"
+    fi; fi], [
+    if test "x$enable_all_plugins" = "xauto"; then
+             if test "x$2" = "xyes"; then
+                     enable_plugin="yes"
+             else
+                     enable_plugin="no"
+             fi
+    else
+             enable_plugin="$enable_all_plugins"
+    fi])
+    if test "x$enable_plugin" = "xyes"; then
+            if test "x$2" = "xyes" || test "x$force" = "xyes"; then
+                    AC_DEFINE([HAVE_PLUGIN_]my_toupper([$1]), 1, [Define to 1 if the $1 plugin is enabled.])
+                    if test "x$2" != "xyes"; then
+                            dependency_warning="yes"
+                    fi
+            else # User passed "yes" but dependency checking yielded "no" => Dependency problem.
+                    dependency_error="yes"
+                    enable_plugin="no (dependency error)"
+            fi
+    fi
+    AM_CONDITIONAL([BUILD_PLUGIN_]my_toupper([$1]), test "x$enable_plugin" = "xyes")
+    enable_$1="$enable_plugin"])# AC_ARG_ENABLE