Switched to a zig build which bundles utf8proc with the lua module.

.gitignore

@@ -6,4 +6,6 @@
 *.lib
 *.def
 .vscode/**
-test/**
+test/**
+zig-cache/**
+zig-out/**

build.zig

@@ -0,0 +1,57 @@
+const std = @import("std");
+
+// Type helpers
+const String     = []const u8;
+const StringPair = [2]String;
+const SourceList = std.ArrayList(String);
+const FlagList   = std.ArrayList(String);
+fn SliceOf(comptime t: type) type {
+    return []const t;
+}
+
+//builder
+const Builder = std.build.Builder;
+const Step    = std.build.Step;
+const InstallDir = std.build.InstallDir;
+const Artifact = *std.build.LibExeObjStep;
+
+pub fn build(b: *Builder) !void {
+    const art = b.addSharedLibrary("lunicode", null, .unversioned);
+    var flags = FlagList.init(b.allocator);
+
+    //Target & build mode
+    const target = b.standardTargetOptions(.{.default_target = std.zig.CrossTarget.fromTarget(b.host.target)});
+    const mode = std.builtin.Mode.ReleaseSmall; // NB. Hardcoded to ReleaseSmall to prevent errors on windows; investigate.
+
+    const luapathopt = b.option(String, "lua", "Path to your lua installation.");
+    if (luapathopt) |luapath| {
+        art.addIncludePath(b.pathJoin(&.{luapath, "include"}));
+        art.addLibraryPath(b.pathJoin(&.{luapath, "lib"}));
+        art.strip = true;
+        art.bundle_compiler_rt = false;
+    } else {
+        std.log.err("No lua path provided!", .{});
+        std.os.exit(1);
+    }
+
+    art.setBuildMode(mode);
+    art.setTarget(target);
+    art.linkLibC();
+    art.linkSystemLibrary("lua5.4.4");
+    art.addIncludePath("utf8proc");
+
+    try flags.append("-DUTF8PROC_STATIC");
+
+    if(target.os_tag == std.Target.Os.Tag.windows) {
+        try flags.append("-DLUA_USE_WINDOWS");
+    }
+    else if ((target.os_tag == std.Target.Os.Tag.linux) or (target.os_tag == std.Target.Os.Tag.macos)) {
+        try flags.append("-DLUA_USE_POSIX");
+        try flags.append("-fPIC");
+    }
+
+    art.addCSourceFiles(&.{"src/lunicode.c", "utf8proc/utf8proc.c"}, flags.toOwnedSlice());
+
+    art.install();
+
+}

src/lunicode.c

@@ -1,12 +1,12 @@
-#include <lua.h>
-#include <luaconf.h>
-#include <lauxlib.h>
 #include <stdbool.h>
 #include <limits.h>
-#include <utf8proc.h>
 #include <stdlib.h>
 #include <stddef.h>
 #include <string.h>
+#include "lua.h"
+#include "luaconf.h"
+#include "lauxlib.h"
+#include "utf8proc.h"
 
 #define all_good(x) ((0 <= (x) && (x) <= 0x10FFFF) && utf8proc_codepoint_valid((x)))
 
@@ -93,8 +93,12 @@ static utf8proc_option_t lunicode_check_options(lua_State* L, int arg) {
 }
 
 static int lunicode_map2(lua_State* L, utf8proc_option_t options) {
-    utf8proc_ssize_t size;
-    const char* str = luaL_checklstring(L, 1, &size);
+    size_t size;
+    utf8proc_uint8_t* str = (utf8proc_uint8_t*)luaL_checklstring(L, 1, &size);
+    utf8proc_ssize_t ssize;
+
+    if (size > PTRDIFF_MAX) ssize = PTRDIFF_MAX;
+    else ssize = size;
 
     utf8proc_ssize_t numwords = utf8proc_decompose(str, size, NULL, 0, options);
 
@@ -108,7 +112,7 @@ static int lunicode_map2(lua_State* L, utf8proc_option_t options) {
 
     utf8proc_ssize_t initial_bytes = numwords * 4;
 
-    char* space = luaL_prepbuffsize(&b, initial_bytes);
+    utf8proc_int32_t* space = (utf8proc_int32_t*)luaL_prepbuffsize(&b, initial_bytes);
 
     numwords = utf8proc_decompose(str, size, space, numwords, options);
 
@@ -185,7 +189,7 @@ static int lunicode_category_string(lua_State* L) {
 static int lunicode_grapheme_break(lua_State* L) {
     lua_Integer i = luaL_checkinteger(L, 1);
     lua_Integer j = luaL_checkinteger(L, 2);
-    lua_Integer s = luaL_checkinteger(L, 3, 0);
+    utf8proc_uint32_t s = (utf8proc_uint32_t)luaL_optinteger(L, 3, 0);
     if (all_good(i) && all_good(j)) {
 
         utf8proc_bool out = utf8proc_grapheme_break_stateful(i, j, &s);
@@ -200,7 +204,7 @@ static int lunicode_grapheme_break(lua_State* L) {
 static int lunicode_properties(lua_State* L) {
     lua_Integer i = luaL_checkinteger(L, 1);
     if (all_good(i)) {
-        utf8proc_property_t* props = utf8proc_get_property(i);
+        const utf8proc_property_t* props = utf8proc_get_property(i);
 
         lua_createtable(L, 0, 10);
 
@@ -261,9 +265,8 @@ static int lunicode_property(lua_State* L) {
         "boundclass",
         "_missing", NULL};
 
-
     if (all_good(i)) {
-        int idx = luaL_checkoption(L, 2, OPTIONS[9], &OPTIONS);
+        int idx = luaL_checkoption(L, 2, OPTIONS[9], OPTIONS);
         utf8proc_property_t* props = utf8proc_get_property(i);
 
         switch (idx) {
@@ -314,7 +317,7 @@ static const luaL_Reg lunicode_methods[] = {
     {NULL, NULL}
 };
 
-LUALIB_API int luaopen_lunicode(lua_State* L) {
+LUA_API int luaopen_lunicode(lua_State* L) {
     luaL_newlib(L, lunicode_methods);
 
     lua_pushliteral(L, "metadata");