Allow disabling unsafe implicit conversions in wxString

While we have to keep these conversions enabled by default, they are very
dangerous as they can result in silent data loss on any system not using a
locale with UTF-8 encoding, i.e. always under MSW.

Allow mitigating this by defining wxNO_UNSAFE_WXSTRING_CONV when compiling the
application code using the library, which makes these conversions invisible to
the user code, and so can be used without recompiling the library.

Also add wxUSE_UNSAFE_WXSTRING_CONV which can be set to 0 when compiling the
library to disable these conversions globally for all applications using it.

Closes #11830.

configure

@@ -1119,6 +1119,7 @@ enable_std_containers_compat
 enable_std_iostreams
 enable_std_string
 enable_std_string_conv_in_wxstring
+enable_unsafe_conv_in_wxstring
 enable_unicode
 enable_utf8
 enable_utf8only
@@ -2057,6 +2058,7 @@ Optional Features:
   --enable-std_iostreams  use standard C++ stream classes
   --enable-std_string     use standard C++ string classes
  --enable-std_string_conv_in_wxstring     provide implicit conversion to std::string in wxString
+ --disable-unsafe_conv_in_wxstring        disable unsafe implicit conversions in wxString
   --disable-unicode       compile without Unicode support
   --enable-utf8           use UTF-8 representation for strings (Unix only)
   --enable-utf8only       only support UTF-8 locales in UTF-8 build (Unix only)
@@ -5469,6 +5471,35 @@ fi
           eval "$wx_cv_use_std_string_conv_in_wxstring"
 
 
+          enablestring=disable
+          defaultval=
+          if test -z "$defaultval"; then
+              if test x"$enablestring" = xdisable; then
+                  defaultval=yes
+              else
+                  defaultval=no
+              fi
+          fi
+
+          # Check whether --enable-unsafe_conv_in_wxstring was given.
+if test "${enable_unsafe_conv_in_wxstring+set}" = set; then :
+  enableval=$enable_unsafe_conv_in_wxstring;
+                          if test "$enableval" = yes; then
+                            wx_cv_use_unsafe_conv_in_wxstring='wxUSE_UNSAFE_WXSTRING_CONV=yes'
+                          else
+                            wx_cv_use_unsafe_conv_in_wxstring='wxUSE_UNSAFE_WXSTRING_CONV=no'
+                          fi
+
+else
+
+                          wx_cv_use_unsafe_conv_in_wxstring='wxUSE_UNSAFE_WXSTRING_CONV=${'DEFAULT_wxUSE_UNSAFE_WXSTRING_CONV":-$defaultval}"
+
+fi
+
+
+          eval "$wx_cv_use_unsafe_conv_in_wxstring"
+
+
           enablestring=disable
           defaultval=
           if test -z "$defaultval"; then
@@ -32527,6 +32558,11 @@ if test "$wxUSE_STD_STRING_CONV_IN_WXSTRING" = "yes"; then
 
 fi
 
+if test "$wxUSE_UNSAFE_WXSTRING_CONV" = "yes"; then
+  $as_echo "#define wxUSE_UNSAFE_WXSTRING_CONV 1" >>confdefs.h
+
+fi
+
 if test "$wxUSE_STDPATHS" = "yes"; then
   $as_echo "#define wxUSE_STDPATHS 1" >>confdefs.h