Implemented lacf_strncasecmp()

• Implemented lacf_strncasecmp() function.
• C99 pedantic conformance flags on apps issue warnings in library parser_funcs.h header.

More compatibility workarounds for using libacfutils from MSVC

• Avoid using VLAs in helpers.h for MSVC compatibility.
• Added fx_lin_multi2 and fx_lin_multi_inv3 function variants, taking explicit point count instead of relying on NULL_VECT2 termination.
• MSVC can't handle typeof or decltype in FILTER_IN, so hard-code to 'double'.

API_EXPORT tags missing on some symbols

API_EXPORT missing on lacf_strlcpy and thread list functionality when using the DLL.

Update to remove useless Brotli dependency & update to latest XPSDK

• Linux build sneaked in a useless dependency on Brotli in the libcurl library. Removed now.
• Updated to latest XPSDK
• safe_strdup() mustn't rely on non-C-standard strdup() function.
• Make log_xplm_cb() only compiled if _LACF_WITHOUT_XPLM isn't defined, to allow linking libacfutils into external apps without XPLM being present.
• Docs tweaks

Update to latest repo state

2 years of updates. See git log for details.
For API documentation, see https://skiselkov.github.io/libacfutils/

airportdb needs to support legacy sceneries without 1302 metainfo lines

• We need to default the airport identifier to be the ICAO code, because legacy sceneries lack metainfo lines.

Removed append_format_buf, as it appears to be broken by certain inputs

• AVG macro should be dividing by a floating point number to take the average.
• append_format_buf appears to be broken by certain inputs, so removed for now, pending refactoring.

Added support for non-ICAO fields and ramp starts in airportdb

• glDrawElements is awfully slow on MacOS if you don't disable GL_VERTEX_ARRAY state first.
• The tmutc2unix function was completely busted and shouldn't exist. Removed.
• Terminate current airport parsing in apt.dat when we hit a seaplane base or heliport.
• Need full versions of IS_NULL_GEO_POS and IS_NULL_VECT macros that check all components.
• Need a macro that automatically zeros out memory before freeing it.
• Refactored airportdb_t to allow for duplicate ICAO codes.
• airportdb_t now has info about ramp starts and IATA codes for airports.

Fixed link problems with lacf_getline on DLL builds

• Don't expose zlib compressed functions, since they can't linked in the DLL build.
• png reading of paletted and grayscale images really needed to be addressed.
• Renamed png_load_from_file_any function to png_load_from_file_rgb_auto, because it really should be doing auto-RGB(A) conversion.

Avoiding extra dependencies on redist builds

• Avoid depending on png.h from glutils.
• Avoid zconf.h depending on unistd.h being present on DLL builds.