Add ability to generate a c header file from scratch, and then both
compile with it and install it if needed.

Example:
```zig
    const avconfig_h = b.addConfigHeader(.{ .path = "libavutil/avconfig.h" }, .generated, .{
        .AV_HAVE_BIGENDIAN = 0, // TODO: detect based on target
        .AV_HAVE_FAST_UNALIGNED = 1, // TODO: detect based on target
    });

    lib.addConfigHeader(avconfig_h);

    lib.installConfigHeader(avconfig_h);
```