cmake: Check for C11 atomics and set config

Context curl#8973

- Also add `threadsafe-init` feature to list.
- Update the `AC_LANG_PROGRAM` to perform the same check as the CMake
  version

CMakeLists.txt

@@ -1055,6 +1055,31 @@ if(HAVE_SIZEOF_LONG_LONG)
   set(HAVE_LL 1)
 endif()
 
+# Check for atomics support
+check_c_source_compiles(
+  "
+  #include <stdatomic.h>
+
+  int main(void)
+  {
+    atomic_int i = 0;
+    return atomic_load(&i);
+  }" HAVE_ATOMIC
+)
+
+# Check for SRW lock support
+if(WIN32)
+  check_c_source_compiles(
+    "
+    #include <windows.h>
+
+    #if (WINVER < 0x600) && (_WIN32_WINNT < 0x600)
+    #error
+    #endif
+    " HAVE_SRW_LOCK
+  )
+endif()
+
 find_file(RANDOM_FILE urandom /dev)
 mark_as_advanced(RANDOM_FILE)
 
@@ -1464,6 +1489,7 @@ _add_if("HTTP3"         USE_NGTCP2 OR USE_QUICHE)
 _add_if("MultiSSL"      CURL_WITH_MULTI_SSL)
 _add_if("HTTPS-proxy"   SSL_ENABLED AND (USE_OPENSSL OR USE_GNUTLS OR USE_NSS))
 _add_if("unicode"       ENABLE_UNICODE)
+_add_if("threadsafe-init" (NOT WIN32 AND HAVE_ATOMIC) OR (HAVE_SRW_LOCK))
 string(REPLACE ";" " " SUPPORT_FEATURES "${_items}")
 message(STATUS "Enabled features: ${SUPPORT_FEATURES}")
 

lib/curl_config.h.cmake

@@ -421,6 +421,9 @@
 /* Define to 1 if the compiler supports the 'long long' data type. */
 #cmakedefine HAVE_LONGLONG 1
 
+/* Define to 1 if the compiler supports C11 atomics. */
+#cmakedefine HAVE_ATOMIC 1
+
 /* Define to 1 if you have the malloc.h header file. */
 #cmakedefine HAVE_MALLOC_H 1
 

m4/curl-functions.m4

@@ -6577,9 +6577,11 @@ AC_DEFUN([CURL_ATOMIC],[
   AC_MSG_CHECKING([if _Atomic is available])
   AC_COMPILE_IFELSE([
     AC_LANG_PROGRAM([[
+      #include <stdatomic.h>
       $curl_includes_unistd
     ]],[[
-      _Atomic int i = 0;
+      atomic_int i = 0;
+      (void) atomic_load(&i);
     ]])
   ],[
     AC_MSG_RESULT([yes])