Support zstd also via pkg-config (#2121)

* Support zstd also via pkg-config

It doesn't always provide cmake config

* Find zstd with pkg-config also in non-required case

Code by @sum01, slightly modified

CMakeLists.txt

@@ -159,10 +159,26 @@ elseif(HTTPLIB_USE_BROTLI_IF_AVAILABLE)
 endif()
 
 if(HTTPLIB_REQUIRE_ZSTD)
-	find_package(zstd REQUIRED)
+	find_package(zstd)
+	if(NOT zstd_FOUND)
+		find_package(PkgConfig REQUIRED)
+		pkg_check_modules(zstd REQUIRED IMPORTED_TARGET libzstd)
+		add_library(zstd::libzstd ALIAS PkgConfig::zstd)
+	endif()
 	set(HTTPLIB_IS_USING_ZSTD TRUE)
 elseif(HTTPLIB_USE_ZSTD_IF_AVAILABLE)
 	find_package(zstd QUIET)
+	if(NOT zstd_FOUND)
+		find_package(PkgConfig QUIET)
+		if(PKG_CONFIG_FOUND)
+			pkg_check_modules(zstd QUIET IMPORTED_TARGET libzstd)
+
+			if(TARGET PkgConfig::zstd)
+				add_library(zstd::libzstd ALIAS PkgConfig::zstd)
+			endif()
+		endif()
+	endif()
+	# Both find_package and PkgConf set a XXX_FOUND var
 	set(HTTPLIB_IS_USING_ZSTD ${zstd_FOUND})
 endif()