Use one C99-style for loop.

Switch one for loop to the new spelling as a canary. All our compilers seem to
support it fine, except GCC needs to be told to build with -std=c99. (And, upon
doing so, it'll require _XOPEN_SOURCE=700 for pthread_rwlock_t.)

We'll let this sit for a bit until it's gotten into downstreams without issue
and then open the floodgates.

BUG=47

Change-Id: I1c69d4b2df8206e0b55f30aa59b5874d82fca893
Reviewed-on: https://boringssl-review.googlesource.com/8235
Reviewed-by: Adam Langley <agl@google.com>

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 3c5db63..bd7f432 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -98,9 +98,18 @@
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wshadow")
 endif()
 
-if((CMAKE_COMPILER_IS_GNUCXX AND CMAKE_C_COMPILER_VERSION VERSION_GREATER "4.8.99") OR
-   CMAKE_CXX_COMPILER_ID MATCHES "Clang")
- set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=c11 -D_XOPEN_SOURCE=700")
+if(CMAKE_COMPILER_IS_GNUCXX)
+  if ((CMAKE_C_COMPILER_VERSION VERSION_GREATER "4.8.99") OR
+      CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=c11")
+  else()
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=c99")
+  endif()
+endif()
+
+# pthread_rwlock_t requires a feature flag.
+if(NOT WIN32)
+  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -D_XOPEN_SOURCE=700")
 endif()
 
 if(FUZZ)

crypto/mem.c

diff --git a/crypto/mem.c b/crypto/mem.c
index 7d73c73..c83eb5a 100644
--- a/crypto/mem.c
+++ b/crypto/mem.c
@@ -118,12 +118,11 @@
 }
 
 int CRYPTO_memcmp(const void *in_a, const void *in_b, size_t len) {
-  size_t i;
   const uint8_t *a = in_a;
   const uint8_t *b = in_b;
   uint8_t x = 0;
 
-  for (i = 0; i < len; i++) {
+  for (size_t i = 0; i < len; i++) {
     x |= a[i] ^ b[i];
   }