Build the crypto libs as C99

Some of the crypto libs rely on UINT64_C, which in glibc 2.17 and earlier was
not defined for C++ code (see
https://sourceware.org/bugzilla/show_bug.cgi?id=15366).

Makefile

@@ -24,7 +24,8 @@ JS_PRE := $(wildcard javascript/*pre.js)
 JS_POST := $(wildcard javascript/*post.js)
 
 CPPFLAGS += -Iinclude -Ilib
-CFLAGS += -Wall -Werror -std=c89 -fPIC
+# we rely on <stdint.h>, which was introduced in C99
+CFLAGS += -Wall -Werror -std=c99 -fPIC
 CXXFLAGS += -Wall -Werror -std=c++11 -fPIC
 LDFLAGS += -Wall -Werror
 

src/libs.cpp → src/libs.c

-/* Copyright 2015 OpenMarket Ltd
+/* Copyright 2015-6 OpenMarket Ltd
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -12,7 +12,6 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-extern "C" {
 #include "crypto-algorithms/sha256.c"
 #include "crypto-algorithms/aes.c"
 #include "curve25519-donna/curve25519-donna.c"
@@ -24,4 +23,3 @@ extern "C" {
 #include "ed25519/src/verify.c"
 #include "ed25519/src/sign.c"
 #include "ed25519_additions.c"
-}