Supports brotli compression

Adds bortli as a submodules, and compiles the decoder, making it available to
libcurl.

Some websites appear to sent brotli encoded responses even though we don't
advertise support for it (e.g. https://movie.douban.com).

.gitmodules

@@ -31,3 +31,6 @@
 [submodule "vendor/curl"]
 	path = vendor/curl
 	url = https://github.com/curl/curl.git
+[submodule "vendor/brotli"]
+	path = vendor/brotli
+	url = https://github.com/google/brotli

build.zig

@@ -245,6 +245,7 @@ fn addDependencies(b: *Build, mod: *Build.Module, opts: *Build.Step.Options) !vo
             mod.addCMacro("HAVE_ASSERT_H", "1");
             mod.addCMacro("HAVE_BASENAME", "1");
             mod.addCMacro("HAVE_BOOL_T", "1");
+            mod.addCMacro("HAVE_BROTLI", "1");
             mod.addCMacro("HAVE_BUILTIN_AVAILABLE", "1");
             mod.addCMacro("HAVE_CLOCK_GETTIME_MONOTONIC", "1");
             mod.addCMacro("HAVE_DLFCN_H", "1");
@@ -379,6 +380,7 @@ fn addDependencies(b: *Build, mod: *Build.Module, opts: *Build.Step.Options) !vo
         }
 
         try buildZlib(b, mod);
+        try buildBrotli(b, mod);
         try buildMbedtls(b, mod);
         try buildNghttp2(b, mod);
         try buildCurl(b, mod);
@@ -484,6 +486,30 @@ fn buildZlib(b: *Build, m: *Build.Module) !void {
     } });
 }
 
+fn buildBrotli(b: *Build, m: *Build.Module) !void {
+    const brotli = b.addLibrary(.{
+        .name = "brotli",
+        .root_module = m,
+    });
+
+    const root = "vendor/brotli/c/";
+    brotli.installHeader(b.path(root ++ "brotli.h"), "brotli.h");
+    brotli.addCSourceFiles(.{ .flags = &.{}, .files = &.{
+        root ++ "common/constants.c",
+        root ++ "common/context.c",
+        root ++ "common/dictionary.c",
+        root ++ "common/platform.c",
+        root ++ "common/shared_dictionary.c",
+        root ++ "common/transform.c",
+        root ++ "dec/bit_reader.c",
+        root ++ "dec/decode.c",
+        root ++ "dec/huffman.c",
+        root ++ "dec/prefix.c",
+        root ++ "dec/state.c",
+        root ++ "dec/static_init.c",
+    } });
+}
+
 fn buildMbedtls(b: *Build, m: *Build.Module) !void {
     const mbedtls = b.addLibrary(.{
         .name = "mbedtls",

src/http/Http.zig

@@ -168,6 +168,13 @@ pub const Connection = struct {
         // debug
         if (comptime Http.ENABLE_DEBUG) {
             try errorCheck(c.curl_easy_setopt(easy, c.CURLOPT_VERBOSE, @as(c_long, 1)));
+
+            // Sometimes the default debug output hides some useful data. You can
+            // uncomment the following line (BUT KEEP THE LIVE ABOVE AS-IS), to
+            // get more control over the data (specifically, the `CURLINFO_TEXT`
+            // can include useful data).
+
+            // try errorCheck(c.curl_easy_setopt(easy, c.CURLOPT_DEBUGFUNCTION, debugCallback));
         }
 
         return .{
@@ -435,3 +442,15 @@ const LineWriter = struct {
         self.col = col + remain.len;
     }
 };
+
+
+pub fn debugCallback(_: *c.CURL, msg_type: c.curl_infotype, raw: [*c]u8, len: usize, _: *anyopaque) callconv(.c) void {
+    const data = raw[0..len];
+    switch (msg_type) {
+        c.CURLINFO_TEXT => std.debug.print("libcurl [text]: {s}\n", .{data}),
+        c.CURLINFO_HEADER_OUT => std.debug.print("libcurl [req-h]: {s}\n", .{data}),
+        c.CURLINFO_HEADER_IN => std.debug.print("libcurl [res-h]: {s}\n", .{data}),
+        // c.CURLINFO_DATA_IN => std.debug.print("libcurl [res-b]: {s}\n", .{data}),
+        else => std.debug.print("libcurl ?? {d}\n", .{msg_type}),
+    }
+}