zig: Add CFLAGS_EXTRA option

This passes through extra options to the compiler. No validation is
performed, the value is passed through as-is.

build.zig

@@ -54,6 +54,8 @@ pub fn build(b: *std.Build) void {
     // TODO: support build_cord
     const build_tests = b.option(bool, "build_tests",
                                  "Build tests") orelse false;
+    const cflags_extra = b.option([]const u8, "CFLAGS_EXTRA",
+                                 "Extra cflags") orelse "";
     // TODO: support enable_docs
     const enable_threads = b.option(bool, "enable_threads",
                 "Support threads") orelse default_enable_threads;
@@ -153,6 +155,8 @@ pub fn build(b: *std.Build) void {
         //"-Wno-long-long",
     }) catch unreachable;
 
+    flags.append(cflags_extra) catch unreachable;
+
     source_files.appendSlice(&.{
         "allchblk.c",
         "alloc.c",