plugin-loader: Add facility to register plugin constructors

Enabled when building monolithically and statically.

This should allow us to work around the -whole-archive issue with
libtool.  If the libraries register the plugin constructors they provide
they reference the constructors and will therefore prevent the linker from
removing these seemingly unused symbols from the final executable.

For use cases where dlsym() can be used, e.g. because the static libraries
are manually linked with -whole-archive (Linux) or -force-load (Apple),
this can be disabled by passing ss_cv_static_plugin_constructors=no to
the configure script.

configure.ac

@@ -1,5 +1,5 @@
 #
-# Copyright (C) 2007-2015 Tobias Brunner
+# Copyright (C) 2007-2017 Tobias Brunner
 # Copyright (C) 2006-2016 Andreas Steffen
 # Copyright (C) 2006-2014 Martin Willi
 # HSR Hochschule fuer Technik Rapperswil
@@ -1303,6 +1303,19 @@ AM_CONDITIONAL(PYTHON_EGGS_INSTALL, [test "x$python_eggs_install" = xtrue])
 
 AM_CONDITIONAL(PERL_CPAN_INSTALL, [test "x$perl_cpan_install" = xtrue])
 
+AC_CACHE_CHECK(
+	[if plugin constructors should be resolved statically],
+	[ss_cv_static_plugin_constructors],
+	[if test x$monolithic = xtrue -a x$enable_static = xyes; then
+		ss_cv_static_plugin_constructors=yes
+	 else
+		ss_cv_static_plugin_constructors="no (enabled for static, monolithic builds)"
+	 fi]
+)
+if test "x$ss_cv_static_plugin_constructors" = xyes; then
+	static_plugin_constructors=true
+fi
+
 # ===============================================
 #  collect plugin list for strongSwan components
 # ===============================================
@@ -1678,6 +1691,7 @@ AM_CONDITIONAL(USE_IMCV, test x$imcv = xtrue)
 AM_CONDITIONAL(USE_TROUSERS, test x$tss_trousers = xtrue)
 AM_CONDITIONAL(USE_TSS2, test x$tss_tss2 = xtrue)
 AM_CONDITIONAL(MONOLITHIC, test x$monolithic = xtrue)
+AM_CONDITIONAL(STATIC_PLUGIN_CONSTRUCTORS, test x$static_plugin_constructors = xtrue)
 AM_CONDITIONAL(USE_SILENT_RULES, test x$enable_silent_rules = xyes)
 AM_CONDITIONAL(COVERAGE, test x$coverage = xtrue)
 AM_CONDITIONAL(USE_DBGHELP, test x$dbghelp_backtraces = xtrue)
@@ -1709,6 +1723,9 @@ fi
 if test x$monolithic = xtrue; then
 	AC_DEFINE([MONOLITHIC], [], [monolithic build embedding plugins])
 fi
+if test x$static_plugin_constructors = xtrue; then
+	AC_DEFINE([STATIC_PLUGIN_CONSTRUCTORS], [], [static plugin constructors])
+fi
 if test x$ikev1 = xtrue; then
 	AC_DEFINE([USE_IKEV1], [], [support for IKEv1 protocol])
 fi