cmake: force C++11 in CHECK_CXX_SOURCE_COMPILES for atomic

GCC 4.8 requires the use of `-std=gnu++11` or similar to enable atomic
features. However, older versions of CMake don't pick up the
project-wide target for C++11 when building the configure check targets.
Although CMake policy 0067 could be set to NEW to enable this, it only
exists on CMake 3.8 and newer, while many of our supported platforms are
on an older version.

cmake/ConfigureChecks.cmake

@@ -189,6 +189,13 @@ int main () {
 FIND_PROGRAM(SED sed)
 
 # https://github.com/fish-shell/fish-shell/issues/5865
+CMAKE_PUSH_CHECK_STATE()
+# Needed until CMP0067 is set to NEW
+# g++ 4.8 needs -std=gnu+11 set to enable atomic features, but CMake < 3.8 does not add that
+# flag to TRY_COMPILE targets even when set for the project
+IF(CMAKE_COMPILER_IS_GNUCXX)
+    LIST(APPEND CMAKE_REQUIRED_FLAGS "-std=gnu++11")
+ENDIF()
 CHECK_CXX_SOURCE_COMPILES("
 #include <atomic>
 #include <cstdint>
@@ -200,3 +207,4 @@ LIBATOMIC_NOT_NEEDED)
 IF (NOT LIBATOMIC_NOT_NEEDED)
     SET(ATOMIC_LIBRARY "atomic")
 ENDIF()
+CMAKE_POP_CHECK_STATE()