conftest: Fix hook constructor resolution via dlsym()

AM_CPPFLAGS only takes preprocessor flags like -I or -D, so it did not
forward -rdynamic to the linker (--export-dynamic), which meant that the
symbols defined in the executable itself were not resolvable via dlsym().

Fixes #394.

src/conftest/Makefile.am

@@ -1,11 +1,13 @@
 ipsec_PROGRAMS = conftest
 
-AM_CPPFLAGS = -rdynamic \
+AM_CPPFLAGS = \
 	-I$(top_srcdir)/src/libstrongswan \
 	-I$(top_srcdir)/src/libhydra \
 	-I$(top_srcdir)/src/libcharon \
 	-DPLUGINS=\""${charon_plugins}\""
 
+AM_CFLAGS = -rdynamic
+
 conftest_SOURCES = conftest.c conftest.h config.c config.h actions.c actions.h \
 	hooks/hook.h hooks/ike_auth_fill.c hooks/unsort_message.c \
 	hooks/add_notify.c hooks/unencrypted_notify.c hooks/ignore_message.c \