Fix Linux build/runtime issues (thread-safety, RSVG, PCRE2, GCC 14, C# SEH, mssql2008 wide-char)

[JesseS-T1]

Summary

• Guard the ft_caches thread-safety reference in fontcache.c so it's only referenced under USE_THREAD
• Escape backslashes inside [...] bracket expressions for PCRE2 (mapcompositingfilter.c, mapregex.c/h) so PCRE2's regex handling matches POSIX regex semantics
• Support inline <?xml SVG symbol data in mapcairo.c, not just file paths for RSVG to support svg legend and scalebar
• Fall back to svg output (not cairopng) when USE_RSVG is the active SVG backend, not just USE_SVG_CAIRO (maplegend.c, mapscale.c)
• Guard MSVC __try/__except SEH usage in csmodule.i behind WIN32 && SWIGCSHARP - GCC has no equivalent and fails to parse it
• Cast renderer_free_func assignments to their declared void (*)(symbolObj *) type, fixing GCC 14's -Wincompatible-pointer-types (promoted to an error by default in GCC 14) in maplegend.c, maprendering.c, mapsymbol.c
• Fix the mssql2008 plugin's USE_ICONV wide-char ODBC path (SQLDriverConnectW/SQLExecDirectW/SQL_WCHAR) assuming a 2-byte wchar_t - true on Windows, false on Linux/glibc's 4-byte wchar_t - which corrupted connection strings, SQL text, and query results on Linux. Restricted that path to _WIN32; other platforms use the existing narrow-char path.

Test plan

• Linux build (bookworm/GCC 12) compiles clean
• mssql2008-backed layers return correct data on Linux (previously returned SQL Server syntax errors from corrupted query text)