Fix SDL_config.h errors on older glibc (#28)

glibc 2.37 and earlier does not support wcslcpy, wcslcat, strlcpy, and
strlcat.

Without implementing a more sophisticated detection, it is simpler to
enable SDL's fallback functions for these often missing routines.

example of affected systems:

Linux Mint 21.3 and earlier
Debian 12 (bookworm) and earlier
Ubuntu 22.10 (kinetic) and earlier
Fedora 37 and earlier
Zorin 17.3 and earlier

Also, updates README.md with some basic instructions.

Author: OrangeTide

README.md

@@ -1,3 +1,32 @@
+# SDL for Zig
+
 This is a fork of [SDL](https://www.libsdl.org/), packaged for [Zig](https://ziglang.org).
 Unnecessary files have been deleted, and the build system has been replaced with `build.zig`.
 The package provides version 2 of SDL. For version 3, consider https://github.com/castholm/SDL.
+
+## Getting started
+
+### Linking SDL2 to your project
+
+Fetch SDL and add to your `build.zig.zon` :
+```bash
+zig fetch --save=SDL git+https://github.com/allyourcodebase/SDL
+```
+
+Add this to your `build.zig` :
+```zig
+pub fn build(b: *std.Build) void {
+// ...
+
+const sdl_dep = b.dependency("SDL", .{
+    .optimize = .ReleaseFast,
+    .target = target,
+});
+
+// ...
+
+exe.linkLibrary(sdl_dep.artifact("SDL2"));
+
+// ...
+}
+```

build.zig

@@ -1087,8 +1087,8 @@ fn configHeader(b: *std.Build, t: std.Target) *std.Build.Step.ConfigHeader {
         .HAVE_MEMMOVE = 1,
         .HAVE_MEMCMP = 1,
         .HAVE_WCSLEN = 1,
-        .HAVE_WCSLCPY = !is_musl,
-        .HAVE_WCSLCAT = !is_musl,
+        .HAVE_WCSLCPY = !is_linux,
+        .HAVE_WCSLCAT = !is_linux,
         .HAVE__WCSDUP = 0,
         .HAVE_WCSDUP = 1,
         .HAVE_WCSSTR = 1,
@@ -1099,8 +1099,8 @@ fn configHeader(b: *std.Build, t: std.Target) *std.Build.Step.ConfigHeader {
         .HAVE_WCSNCASECMP = 1,
         .HAVE__WCSNICMP = 0,
         .HAVE_STRLEN = 1,
-        .HAVE_STRLCPY = 1,
-        .HAVE_STRLCAT = 1,
+        .HAVE_STRLCPY = !is_linux or is_musl,
+        .HAVE_STRLCAT = !is_linux or is_musl,
         .HAVE__STRREV = 0,
         .HAVE__STRUPR = 0,
         .HAVE__STRLWR = 0,