Backport of two upstream build fixes needed for Firefox's hazard build
(GCC + libstdc++ 10 sysroot + C++20 + -fno-rtti):
- dynamic_cast -> static_cast in affixmgr.cxx
  Upstream PR: https://github.com/hunspell/hunspell/pull/1124 (merged)
- Gate std::bit_cast on __cpp_lib_bit_cast in csutil.cxx / hashmgr.cxx
  Upstream PR: https://github.com/hunspell/hunspell/pull/1125 (merged)
Both will land naturally on the next vendor bump; this patch can be dropped then.

--- a/src/affixmgr.cxx
+++ b/src/affixmgr.cxx
@@ -4697,9 +4697,9 @@ bool AffixMgr::parse_affix(const std::string& line,
   auto start = affentries.begin(), end = affentries.end();
   for (auto affentry = start; affentry != end; ++affentry) {
     if (at == 'P') {
-      build_pfxtree(dynamic_cast<PfxEntry*>(*affentry));
+      build_pfxtree(static_cast<PfxEntry*>(*affentry));
     } else {
-      build_sfxtree(dynamic_cast<SfxEntry*>(*affentry));
+      build_sfxtree(static_cast<SfxEntry*>(*affentry));
     }
   }

--- a/src/csutil.cxx
+++ b/src/csutil.cxx
@@ -559,7 +559,7 @@ w_char upper_utf(w_char u, int langnum) {
 //but g++ remains in five instructions
 //maybe use inline asm for g++?

-#if __cplusplus >= 202002L || (defined(_MSVC_LANG) && _MSVC_LANG >= 202002L)
+#if (__cplusplus >= 202002L || (defined(_MSVC_LANG) && _MSVC_LANG >= 202002L)) && defined __cpp_lib_bit_cast && __cpp_lib_bit_cast >= 201806L
   return std::bit_cast<w_char>(unicodetoupper((unsigned short)u, langnum));
 #else
   const auto us = unicodetoupper((unsigned short)u, langnum);
@@ -583,7 +583,7 @@ w_char lower_utf(w_char u, int langnum) {
 //but g++ remains in five instructions
 //maybe use inline asm for g++?

-#if __cplusplus >= 202002L || (defined(_MSVC_LANG) && _MSVC_LANG >= 202002L)
+#if (__cplusplus >= 202002L || (defined(_MSVC_LANG) && _MSVC_LANG >= 202002L)) && defined __cpp_lib_bit_cast && __cpp_lib_bit_cast >= 201806L
   return std::bit_cast<w_char>(unicodetolower((unsigned short)u, langnum));
 #else
   const auto us = unicodetolower((unsigned short)u, langnum);
--- a/src/hashmgr.cxx
+++ b/src/hashmgr.cxx
@@ -983,7 +983,7 @@ std::string HashMgr::encode_flag(unsigned short f) const {

 #if defined(_WIN32) || (defined(__BYTE_ORDER__) && (__BYTE_ORDER__==__ORDER_LITTLE_ENDIAN__))  || defined(__LITTLE_ENDIAN__)

-#if __cplusplus >= 202002L || (defined(_MSVC_LANG) && _MSVC_LANG >= 202002L)
+#if (__cplusplus >= 202002L || (defined(_MSVC_LANG) && _MSVC_LANG >= 202002L)) && defined __cpp_lib_bit_cast && __cpp_lib_bit_cast >= 201806L
     auto wc = std::bit_cast<w_char>(f);
 #else
     w_char wc;