amount: make it work with gcc-4.8.

```
In file included from bitcoin/chainparams.h:7:0,from bitcoin/chainparams.c:1:
./common/amount.h:36:11: error: initializer element is not constant
((struct amount_sat){(constant) + AMOUNT_MUST_BE_CONST(constant)})
^
bitcoin/chainparams.c:20:21: note: in expansion of macro ‘AMOUNT_SAT’
.max_funding = AMOUNT_SAT((1 << 24) - 1),
^
./common/amount.h:36:11: error: (near initialization for ‘networks[0].max_funding’)
((struct amount_sat){(constant) + AMOUNT_MUST_BE_CONST(constant)})
^
bitcoin/chainparams.c:20:21: note: in expansion of macro ‘AMOUNT_SAT’
.max_funding = AMOUNT_SAT((1 << 24) - 1),
```

Fixes: #2404
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

bitcoin/chainparams.c

@@ -17,8 +17,8 @@ const struct chainparams networks[] = {
       *...
       *   - MUST set `funding_satoshis` to less than 2^24 satoshi.
       */
-     .max_funding = AMOUNT_SAT((1 << 24) - 1),
-     .max_payment = AMOUNT_MSAT(0xFFFFFFFFULL),
+     .max_funding = AMOUNT_SAT_INIT((1 << 24) - 1),
+     .max_payment = AMOUNT_MSAT_INIT(0xFFFFFFFFULL),
      /* "Lightning Charge Powers Developers & Blockstream Store" */
      .when_lightning_became_cool = 504500,
      .testnet = false},
@@ -29,8 +29,8 @@ const struct chainparams networks[] = {
      .cli = "bitcoin-cli",
      .cli_args = "-regtest",
      .dust_limit = { 546 },
-     .max_funding = AMOUNT_SAT((1 << 24) - 1),
-     .max_payment = AMOUNT_MSAT(0xFFFFFFFFULL),
+     .max_funding = AMOUNT_SAT_INIT((1 << 24) - 1),
+     .max_payment = AMOUNT_MSAT_INIT(0xFFFFFFFFULL),
      .when_lightning_became_cool = 1,
      .testnet = true},
     {.network_name = "testnet",
@@ -40,8 +40,8 @@ const struct chainparams networks[] = {
      .cli = "bitcoin-cli",
      .cli_args = "-testnet",
      .dust_limit = { 546 },
-     .max_funding = AMOUNT_SAT((1 << 24) - 1),
-     .max_payment = AMOUNT_MSAT(0xFFFFFFFFULL),
+     .max_funding = AMOUNT_SAT_INIT((1 << 24) - 1),
+     .max_payment = AMOUNT_MSAT_INIT(0xFFFFFFFFULL),
      .testnet = true},
     {.network_name = "litecoin",
      .bip173_name = "ltc",
@@ -50,8 +50,8 @@ const struct chainparams networks[] = {
      .cli = "litecoin-cli",
      .cli_args = NULL,
      .dust_limit = { 100000 },
-     .max_funding = AMOUNT_SAT(60 * ((1 << 24) - 1)),
-     .max_payment = AMOUNT_MSAT(60 * 0xFFFFFFFFULL),
+     .max_funding = AMOUNT_SAT_INIT(60 * ((1 << 24) - 1)),
+     .max_payment = AMOUNT_MSAT_INIT(60 * 0xFFFFFFFFULL),
      .when_lightning_became_cool = 1320000,
      .testnet = false},
     {.network_name = "litecoin-testnet",
@@ -61,8 +61,8 @@ const struct chainparams networks[] = {
      .cli = "litecoin-cli",
      .cli_args = "-testnet",
      .dust_limit = { 100000 },
-     .max_funding = AMOUNT_SAT(60 * ((1 << 24) - 1)),
-     .max_payment = AMOUNT_MSAT(60 * 0xFFFFFFFFULL),
+     .max_funding = AMOUNT_SAT_INIT(60 * ((1 << 24) - 1)),
+     .max_payment = AMOUNT_MSAT_INIT(60 * 0xFFFFFFFFULL),
      .when_lightning_became_cool = 1,
      .testnet = true}
 };

common/amount.h

@@ -29,6 +29,13 @@ struct amount_msat {
 #define AMOUNT_MUST_BE_CONST(c) 0
 #endif
 
+/* GCC 4.8.5 (Centos 7.6!) thinks struct casts are not constants, so we
+ * need to not use a cast for static initializations. */
+#define AMOUNT_MSAT_INIT(msat)		\
+	{ .millisatoshis = (msat) }
+#define AMOUNT_SAT_INIT(sat)		\
+	{ .satoshis = (sat) }
+
 #define AMOUNT_MSAT(constant)						\
 	((struct amount_msat){(constant) + AMOUNT_MUST_BE_CONST(constant)})
 

onchaind/onchaind.c

@@ -156,7 +156,7 @@ static bool set_htlc_timeout_fee(struct bitcoin_tx *tx,
 				 const struct bitcoin_signature *remotesig,
 				 const u8 *wscript)
 {
-	static struct amount_sat fee = AMOUNT_SAT(UINT64_MAX);
+	static struct amount_sat fee = AMOUNT_SAT_INIT(UINT64_MAX);
 
 	/* BOLT #3:
 	 *
@@ -181,7 +181,7 @@ static void set_htlc_success_fee(struct bitcoin_tx *tx,
 				 const struct bitcoin_signature *remotesig,
 				 const u8 *wscript)
 {
-	static struct amount_sat fee = AMOUNT_SAT(UINT64_MAX);
+	static struct amount_sat fee = AMOUNT_SAT_INIT(UINT64_MAX);
 
 	/* BOLT #3:
 	 *