[zig] Fix build.zig bug

[VisenDev]

Before this change, building raylib using the config options field fails.

    const ray = b.dependency("raylib", .{
        .target = target,
        .optimize = optimize,
        .config = @as([]const u8, "-DSUPPORT_CUSTOM_FRAME_CONTROL"),
    });

Robert@Roberts-MacBook-Pro-2 ~/z/dev (master)> zig build
thread 1133588 panic: attempt to use null value
/Users/Robert/.cache/zig/p/1220cda9f85d36e6f5be4bac65430b056c4e3d4ab3fafcf71c73ec6b1853e8a20856/src/build.zig:81:58: 0x10a8dfba5 in compileRaylib (build)
            flag = flag[0..std.mem.indexOf(u8, flag, " ").?]; // Flag is only one word, so capture till space
                                                         ^
/Users/Robert/.cache/zig/p/1220cda9f85d36e6f5be4bac65430b056c4e3d4ab3fafcf71c73ec6b1853e8a20856/src/build.zig:369:34: 0x10a8e5219 in build (build)
    const lib = try compileRaylib(b, target, optimize, options);
                                 ^
/Users/Robert/.cache/zig/p/1220cda9f85d36e6f5be4bac65430b056c4e3d4ab3fafcf71c73ec6b1853e8a20856/build.zig:6:21: 0x10a8e6af4 in build (build)
    try raylib.build(b);
                    ^
/usr/local/Cellar/zig/0.13.0/lib/zig/std/Build.zig:2117:43: 0x10a86c484 in runBuild__anon_15294 (build)
        .ErrorUnion => try build_zig.build(b),
                                          ^
/usr/local/Cellar/zig/0.13.0/lib/zig/std/Build.zig:2097:29: 0x10a823ab9 in dependencyInner__anon_14532 (build)
        sub_builder.runBuild(bz) catch @panic("unhandled error");
                            ^
/usr/local/Cellar/zig/0.13.0/lib/zig/std/Build.zig:1954:35: 0x10a7d7c00 in dependency__anon_11228 (build)
            return dependencyInner(b, name, pkg.build_root, if (@hasDecl(pkg, "build_zig")) pkg.build_zig else null, pkg_hash, pkg.deps, args);
                                  ^
/Users/Robert/zig/dev/build.zig:98:29: 0x10a7d2f68 in build (build)
    const ray = b.dependency("raylib", .{
                            ^
/usr/local/Cellar/zig/0.13.0/lib/zig/std/Build.zig:2117:43: 0x10a7b58c0 in runBuild__anon_8431 (build)
        .ErrorUnion => try build_zig.build(b),
                                          ^
/usr/local/Cellar/zig/0.13.0/lib/zig/compiler/build_runner.zig:301:29: 0x10a7b0424 in main (build)
        try builder.runBuild(root);
                            ^
/usr/local/Cellar/zig/0.13.0/lib/zig/std/start.zig:524:37: 0x10a7b808b in main (build)
            const result = root.main() catch |err| {
                                    ^
???:?:?: 0x7ff8049af30f in ??? (???)
Unwind information for `???:0x7ff8049af30f` was not available, trace may be incomplete

???:?:?: 0x0 in ??? (???)
error: the following build command crashed:
/Users/Robert/zig/dev/.zig-cache/o/1e0d00ce3951a8ebfb69b30e398dd680/build /usr/local/Cellar/zig/0.13.0/bin/zig /Users/Robert/zig/dev /Users/Robert/zig/dev/.zig-cache /Users/Robert/.cache/zig --seed 0xd202b3c9 -Z7e6f5d7f6781950c

The code get broken by this line in config.h

#ifdef RL_SUPPORT_MESH_GPU_SKINNING

To fix this, I made the following changes

• build.zig now skips processing any config.h lines that start with #if,
• build.zig now continues rather than panics if std.mem.indexOf(u8, flag, " ") fails.

I also added a comment that the config option should be a list of cflags, since I had initially assumed this option was meant to be a path to a custom config.h file. I didn't realize my mistake until reading thru the build code very closely, so a clarifying comment should help explain how this field ought to be properly used.

[VisenDev]

To reproduce this bug, you can run the following command in the raylib master branch.

zig build -Dconfig="-DSUPPORT_CUSTOM_FRAME_CONTROL"

Tested with both zig 0.12.0 and zig 0.13.0

[raysan5]

@VisenDev I'm merging it but note I don't maintain the buld.zig system so it could fail for other users.