Index: configure.ac
==================================================================
--- configure.ac
+++ configure.ac
@@ -66,10 +66,32 @@
 	AC_SUBST(PROPERTIESTESTS_M, "PropertiesTests.m")
 	AC_MSG_RESULT(yes)
 ], [
 	AC_MSG_RESULT(no)
 ])
+
+AC_MSG_CHECKING(whether Objective C compiler supports optional protocols)
+AC_TRY_COMPILE([
+	@protocol Proto
+	@optional
+	+ (void)foo;
+	@end
+
+	@interface Foo <Proto>
+	@end
+
+	@implementation Foo
+	@end
+], [
+	[Foo foo];
+], [
+	AC_DEFINE(OF_HAVE_OPTIONAL_PROTOCOLS, 1,
+		[Compiler support for optional protocols])
+	AC_MSG_RESULT(yes)
+], [
+	AC_MSG_RESULT(no)
+])
 
 AC_MSG_CHECKING(whether Objective C compiler supports blocks)
 old_OBJCFLAGS="$OBJCFLAGS"
 OBJCFLAGS="$OBJCFLAGS -fblocks"
 AC_TRY_COMPILE([], [

Index: src/objfw-defs.h.in
==================================================================
--- src/objfw-defs.h.in
+++ src/objfw-defs.h.in
@@ -4,10 +4,11 @@
 #undef OF_GNU_RUNTIME
 #undef OF_HAVE_ASPRINTF
 #undef OF_HAVE_BLOCKS
 #undef OF_HAVE_FAST_ENUMERATION
 #undef OF_HAVE_GCC_ATOMIC_OPS
+#undef OF_HAVE_OPTIONAL_PROTOCOLS
 #undef OF_HAVE_OSATOMIC
 #undef OF_HAVE_OSATOMIC_64
 #undef OF_HAVE_POLL
 #undef OF_HAVE_PROPERTIES
 #undef OF_HAVE_PTHREADS