Add C "contract" test

Tests that the `%contract` code (viz., each target language's
implementation of SWIG_contract_assert) compiles in C as expected. Update
the note in the source code since `%contract` is in the official
documentation.

Examples/test-suite/common.mk

@@ -695,6 +695,7 @@ C_TEST_CASES += \
 	command_line_define \
 	const_const \
 	constant_expr_c \
+	contract_c \
 	default_args_c \
 	empty_c \
 	enums \

Examples/test-suite/contract.i

@@ -48,6 +48,7 @@ int test_prepost(int x, int y) {
 }
 %}
 
+#ifdef __cplusplus
 /* Class tests */
 
 %contract Foo::test_preassert(int x, int y) {
@@ -235,4 +236,4 @@ class myClass
 };
 
 }
-
+#endif

Examples/test-suite/contract_c.i

@@ -0,0 +1,5 @@
+%module contract_c;
+
+%include <exception.i>
+
+%include "contract.i"

Lib/swig.swg

@@ -172,7 +172,7 @@
 #define %novaluewrapper      %feature("novaluewrapper")
 #define %clearnovaluewrapper %feature("novaluewrapper","")
 
-/* Contract support - Experimental and undocumented */
+/* Contract support - Experimental */
 #define %contract      %feature("contract")
 #define %nocontract    %feature("contract","0")
 #define %clearcontract %feature("contract","")