content_encoding: add zstd decoding support

include zstd curl patch for Makefile.m32 from vszakats
and include Add CMake support for zstd from Peter Wu

Helped-by: Viktor Szakats
Helped-by: Peter Wu
Closes #5453

docs/examples/Makefile.m32

@@ -24,7 +24,7 @@
 #
 ## Makefile for building curl examples with MingW (GCC-3.2 or later)
 ## and optionally OpenSSL (1.0.2a), libssh2 (1.5), zlib (1.2.8), librtmp (2.4),
-## brotli (1.0.1)
+## brotli (1.0.1), zstd (1.4.5)
 ##
 ## Usage:   mingw32-make -f Makefile.m32 CFG=-feature1[-feature2][-feature3][...]
 ## Example: mingw32-make -f Makefile.m32 CFG=-zlib-ssl-sspi-winidn
@@ -39,6 +39,10 @@
 ifndef ZLIB_PATH
 ZLIB_PATH = ../../../zlib-1.2.8
 endif
+# Edit the path below to point to the base of your Zstandard sources.
+ifndef ZSTD_PATH
+ZSTD_PATH = ../../../zstd-1.4.5
+endif
 # Edit the path below to point to the base of your Brotli sources.
 ifndef BROTLI_PATH
 BROTLI_PATH = ../../../brotli-1.0.1
@@ -180,6 +184,9 @@ endif
 ifeq ($(findstring -zlib,$(CFG)),-zlib)
 ZLIB = 1
 endif
+ifeq ($(findstring -zstd,$(CFG)),-zstd)
+ZSTD = 1
+endif
 ifeq ($(findstring -brotli,$(CFG)),-brotli)
 BROTLI = 1
 endif
@@ -284,6 +291,11 @@ ifdef ZLIB
   CFLAGS += -DHAVE_LIBZ -DHAVE_ZLIB_H
   curl_LDADD += -L"$(ZLIB_PATH)" -lz
 endif
+ifdef ZSTD
+  INCLUDES += -I"$(ZSTD_PATH)/include"
+  CFLAGS += -DHAVE_ZSTD
+  curl_LDADD += -L"$(ZSTD_PATH)/lib" -lzstd
+endif
 ifdef BROTLI
   INCLUDES += -I"$(BROTLI_PATH)/include"
   CFLAGS += -DHAVE_BROTLI

docs/libcurl/curl_version_info.3

@@ -92,6 +92,11 @@ typedef struct {
                                   be NULL */
   const char *capath;          /* the built-in default CURLOPT_CAPATH, might
                                   be NULL */
+  /* when 'age' is CURLVERSION_EIGHTH or higher (>= 7.71.0), the members
+     below exist */
+  unsigned int zstd_ver_num; /* Numeric Zstd version
+                                  (MAJOR << 24) | (MINOR << 12) | PATCH */
+  const char *zstd_version; /* human readable string. */
 } curl_version_info_data;
 .fi
 
@@ -121,6 +126,8 @@ more exact timeouts (even on Windows) and less blocking when using the multi
 interface. (added in 7.10.7)
 .IP CURL_VERSION_BROTLI
 supports HTTP Brotli content encoding using libbrotlidec (Added in 7.57.0)
+.IP CURL_VERSION_ZSTD
+supports HTTP zstd content encoding using zstd library (Added in 7.72.0)
 .IP CURL_VERSION_CONV
 libcurl was built with support for character conversions, as provided by the
 CURLOPT_CONV_* callbacks. (Added in 7.15.4)

docs/libcurl/opts/CURLOPT_ACCEPT_ENCODING.3

@@ -45,8 +45,9 @@ Alternatively, you can specify exactly the encoding or list of encodings you
 want in the response. Four encodings are supported: \fIidentity\fP, meaning
 non-compressed, \fIdeflate\fP which requests the server to compress its
 response using the zlib algorithm, \fIgzip\fP which requests the gzip
-algorithm and (since curl 7.57.0) \fIbr\fP which is brotli.  Provide them in
-the string as a comma-separated list of accepted encodings, like:
+algorithm, (since curl 7.57.0) \fIbr\fP which is brotli and (since curl
+7.72.0) \fIzstd\fP which is zstd.  Provide them in the string as a
+comma-separated list of accepted encodings, like:
 
   "br, gzip, deflate".
 
@@ -94,8 +95,9 @@ if(curl) {
 This option was called CURLOPT_ENCODING before 7.21.6
 
 The specific libcurl you're using must have been built with zlib to be able to
-decompress gzip and deflate responses and with the brotli library to
-decompress brotli responses.
+decompress gzip and deflate responses, with the brotli library to
+decompress brotli responses and with the zstd library to decompress zstd
+responses.
 .SH RETURN VALUE
 Returns CURLE_OK if the option is supported, CURLE_UNKNOWN_OPTION if not, or
 CURLE_OUT_OF_MEMORY if there was insufficient heap space.

docs/libcurl/symbols-in-versions

@@ -803,6 +803,7 @@ CURLU_NO_DEFAULT_PORT           7.62.0
 CURLU_PATH_AS_IS                7.62.0
 CURLU_URLDECODE                 7.62.0
 CURLU_URLENCODE                 7.62.0
+CURLVERSION_EIGHTH              7.72.0
 CURLVERSION_FIFTH               7.57.0
 CURLVERSION_FIRST               7.10
 CURLVERSION_FOURTH              7.16.1
@@ -968,6 +969,7 @@ CURL_VERSION_SSL                7.10
 CURL_VERSION_SSPI               7.13.2
 CURL_VERSION_TLSAUTH_SRP        7.21.4
 CURL_VERSION_UNIX_SOCKETS       7.40.0
+CURL_VERSION_ZSTD               7.72.0
 CURL_WAIT_POLLIN                7.28.0
 CURL_WAIT_POLLOUT               7.28.0
 CURL_WAIT_POLLPRI               7.28.0