cgo: add support for #define constants

These are converted to Go constants where possible.

testdata/cgo/main.go

@@ -18,6 +18,10 @@ func main() {
 	var y C.longlong = -(1 << 40)
 	println("longlong:", y)
 	println("global:", C.global)
+	println("defined ints:", C.CONST_INT, C.CONST_INT2)
+	println("defined floats:", C.CONST_FLOAT, C.CONST_FLOAT2)
+	println("defined string:", C.CONST_STRING)
+	println("defined char:", C.CONST_CHAR)
 	var ptr C.intPointer
 	var n C.int = 15
 	ptr = C.intPointer(&n)

testdata/cgo/main.h

@@ -10,6 +10,13 @@ int doCallback(int a, int b, binop_t cb);
 typedef int * intPointer;
 void store(int value, int *ptr);
 
+# define CONST_INT 5
+# define CONST_INT2 5llu
+# define CONST_FLOAT 5.8
+# define CONST_FLOAT2 5.8f
+# define CONST_CHAR 'c'
+# define CONST_STRING "defined string"
+
 // this signature should not be included by CGo
 void unusedFunction2(int x, __builtin_va_list args);
 

testdata/cgo/out.txt

@@ -4,6 +4,10 @@ myint: 3 5
 myint size: 2
 longlong: -1099511627776
 global: 3
+defined ints: 5 5
+defined floats: +5.800000e+000 +5.800000e+000
+defined string: defined string
+defined char: 99
 15: 15
 25: 25
 callback 1: 50