cgo: add support for CFLAGS in .c files

This patch adds support for passing CFLAGS added in #cgo lines of the
CGo preprocessing phase to the compiler when compiling C files inside
packages. This is expected and convenient but didn't work before.

testdata/cgo/main.h

@@ -139,6 +139,8 @@ extern bitfield_t globalBitfield;
 
 extern int smallEnumWidth;
 
+extern int cflagsConstant;
+
 // test duplicate definitions
 int add(int a, int b);
 extern int global;