Hide or fix all build warnings (#1331)

* Changing C-strings to be const.

* Changed variable-length arrays to std::vector to avoid warnings. VLA is a compiler extension.

* Changed `#define` inside functions into `constexpr int` to preserve scoping and avoid macro redefinition warnings.

* Disabled warnings for modifying `CMAKE_CXX_FLAGS` caused by `check_symbol_exists`, which temporarily modifies the flag to do a compile check.

* Fixed VLA in rccl UT.

test/AllToAllTests.cpp

@@ -100,7 +100,7 @@ namespace RcclUnitTesting
         std::vector<bool>           const inPlaceList     = {false};
         std::vector<bool>           const managedMemList  = {false};
         std::vector<bool>           const useHipGraphList = {false, true};
-        std::vector<char *>         const channelList     = {"112"}; 
+        std::vector<const char *>   const channelList     = {"112"}; 
         bool                        const enableSweep     = false;
         for (auto channel : channelList) {
           setenv("NCCL_MIN_NCHANNELS", channel, 1);