Allow event/function definitions to be wrapped in directives.

This makes

@if (conditions)
event a(...)
@else
event b(...)
@endif

work, which threw an error in the past. This is useful when event
definition change in newer Bro version and code wants to accept both
kinds of events.

src/parse.y

@@ -1142,9 +1142,16 @@ decl:
 	|	func_hdr func_body
 			{ }
 
+	|	func_hdr conditional_list func_body
+			{ }
+
 	|	conditional
 	;
 
+conditional_list:
+		conditional
+	|	conditional conditional_list
+
 conditional:
 		TOK_ATIF '(' expr ')'
 			{ do_atif($3); }

testing/btest/Baseline/language.at-if-event/out

@@ -0,0 +1,3 @@
+1
+2
+3

testing/btest/language/at-if-event.bro

@@ -0,0 +1,31 @@
+# @TEST-EXEC: bro -b %INPUT >out
+# @TEST-EXEC: btest-diff out
+# Check if @if can be used to alternative function/event definitions
+
+@if ( 1==1 )
+function test_case(msg: string)
+@else
+lalala
+@endif
+	{
+	print msg;
+	}
+
+@if ( 1==1 )
+event bro_init()
+@else
+lalala
+@endif
+	{
+	print "1";
+	test_case("2");
+	}
+
+@if ( 1==0 )
+lalala
+@else
+event bro_init()
+@endif
+	{
+	print "3";
+	}