Add a configure option to disable all default plugins

The --disable-defaults option disables all plugins that would be
enabled by default.  This allows to selectively enable specific plugins
without issues when new default options get added in future releases.

configure.in

@@ -230,6 +230,19 @@ ARG_ENABL_SET([vstr],           [enforce using the Vstr string library to replac
 ARG_ENABL_SET([monolithic],     [build monolithic version of libstrongswan that includes all enabled plugins. Similarly, the plugins of charon are assembled in libcharon.])
 ARG_ENABL_SET([bfd-backtraces], [use binutils libbfd to resolve backtraces for memory leaks and segfaults.])
 
+dnl ===================================
+dnl  option to disable default options
+dnl ===================================
+
+ARG_DISBL_SET([defaults],       [disable all default plugins (they can be enabled with their respective --enable options)])
+
+if test x$defaults = xfalse; then
+	for option in $enabled_by_default; do
+		eval test x\${${option}_given} = xtrue && continue
+		let $option=false
+	done
+fi
+
 dnl =========================
 dnl  set up compiler and flags
 dnl =========================
@@ -994,7 +1007,6 @@ AC_SUBST(medsrv_plugins)
 AC_SUBST(nm_plugins)
 
 AC_SUBST(c_plugins)
-AC_SUBST(p_plugins)
 AC_SUBST(h_plugins)
 AC_SUBST(s_plugins)
 

m4/macros/enable-disable.m4

@@ -20,6 +20,7 @@ AC_DEFUN([ARG_ENABL_SET],
 # ARG_DISBL_SET(option, help)
 # ---------------------------
 # Create a --disable-$1 option with helptext, set a variable $1 to true/false
+# All $1 are collected in the variable $enabled_by_default
 AC_DEFUN([ARG_DISBL_SET],
 	[AC_ARG_ENABLE(
 		[$1],
@@ -32,5 +33,6 @@ AC_DEFUN([ARG_DISBL_SET],
 		fi],
 		[patsubst([$1], [-], [_])=true
 		patsubst([$1], [-], [_])_given=false]
-	)]
+	)
+	enabled_by_default=${enabled_by_default}" patsubst([$1], [-], [_])"]
 )