flag,json,net: handle C calls in .v files (part of enabling -W impure-v as default) (#19779)

vlib/net/openssl/openssl_compiles_test.c.v

@@ -0,0 +1,17 @@
+import net.openssl
+
+struct Abc {
+	x &C.SSL_CTX
+}
+
+fn test_printing_struct_with_reference_field_of_type_ssl_ctx() {
+	a := unsafe { Abc{&C.SSL_CTX(123)} }
+	dump(a)
+	sa := a.str()
+	assert sa.contains('&C.SSL_CTX(0x7b)')
+}
+
+fn test_openssl_compiles() {
+	assert openssl.is_used == 1
+	assert true
+}