cmake: openssl: Added check for atomic library

OpenSSL on arm7 utilized atomics. In order for the configuration check
for HMAC_CTX_new to succeed, the atomic library needs to be added
to CMAKE_REQUIRED_LIBRARIES.

Fix for #3529

Author: mprather

lib/tls/CMakeLists.txt

@@ -333,8 +333,16 @@ endif()
 if (DEFINED LIB_LIST)
 	set(CMAKE_REQUIRED_LIBRARIES ${LIB_LIST})
 endif()
+if(UNIX AND (CMAKE_C_COMPILER_ID STREQUAL "GNU" OR CMAKE_C_COMPILER_ID MATCHES "Clang"))
+  check_c_source_compiles("int main() { long long x = 0; __atomic_fetch_add(&x, 1, __ATOMIC_SEQ_CST); return 0; }" HAVE_WORKING_ATOMIC_64)
+  if(HAVE_WORKING_ATOMIC_64)
+    set(LWS_ATOMIC_LIB "")
+  else()
+    set(LWS_ATOMIC_LIB atomic)
+  endif()
+endif()
 if (UNIX AND NOT (${CMAKE_SYSTEM_NAME} MATCHES "QNX"))
-	set(CMAKE_REQUIRED_LIBRARIES ${CMAKE_REQUIRED_LIBRARIES} ${CMAKE_DL_LIBS})
+	set(CMAKE_REQUIRED_LIBRARIES ${CMAKE_REQUIRED_LIBRARIES} ${CMAKE_DL_LIBS} ${LWS_ATOMIC_LIB})
 endif()
 if ((CMAKE_COMPILER_IS_GNUCC OR CMAKE_COMPILER_IS_GNUCXX) AND NOT ((${CMAKE_SYSTEM_NAME} MATCHES "QNX") OR PC_OPENSSL_FOUND))
 	set(CMAKE_REQUIRED_LIBRARIES ${CMAKE_REQUIRED_LIBRARIES} pthread)