Clean up Jansson source tree, document changes made

* Update jansson_private_config.h to accurately reflect the presence or
  absence of certain compiler features and headers
* Mark one function as unused to prevent a compilation warning
* Disable one warning on GCC
* Validate that all imported C files are used in the build
* Remove unused Jansson build files, preserve LICENSE
* Document these changes in WEBDIS-CHANGES.md and link from README.md

README.md

@@ -3,13 +3,13 @@
 
 # About Webdis
 
-A very simple web server providing an HTTP interface to Redis. It uses [hiredis](https://github.com/antirez/hiredis), [jansson](https://github.com/akheron/jansson), [libevent](https://monkey.org/~provos/libevent/), and [http-parser](https://github.com/ry/http-parser/).
+A very simple web server providing an HTTP interface to Redis. It embeds [hiredis](https://github.com/antirez/hiredis), [jansson](https://github.com/akheron/jansson) (with some [local changes](./src/jansson/WEBDIS-CHANGES.md)), and [http-parser](https://github.com/ry/http-parser/). It also depends on [libevent](https://monkey.org/~provos/libevent/), to be installed separately.
 
-Webdis depends on libevent-dev. You can install it on Ubuntu by typing `sudo apt-get install libevent-dev` or on macOS by typing `brew install libevent`.
+# Build and run from source
 
-To build Webdis with support for encrypted connections to Redis, see [Building Webdis with SSL support](#building-webdis-with-ssl-support).
+Building Webdis requires the libevent development package. You can install it on Ubuntu by typing `sudo apt-get install libevent-dev` or on macOS by typing `brew install libevent`.
 
-# Build and run from source
+To build Webdis with support for encrypted connections to Redis, see [Building Webdis with SSL support](#building-webdis-with-ssl-support).
 
 ```sh
 $ make clean all

src/jansson/WEBDIS-CHANGES.md

@@ -0,0 +1,37 @@
+# Webdis changes to Jansson
+
+Webdis imports Jansson by including its source code under `src/jansson`. This document lists the changes made to Jansson files, mostly made to accommodate the requirements of the Webdis build process.
+
+Webdis currently imports Jansson version 2.14.
+
+## Includes and compiler builtins
+
+Added checks for various headers and compiler builtins. For example, `endian.h` is not available on macOS, although `machine/endian.h` is.
+
+## Unused code
+
+Marked `buf_to_uint32` as unused.
+
+## Unused source files
+
+To validate that all the C files imported are used in the build, we can compare the list of files in the Jansson source directory with the object files listed in the Webdis Makefile.
+
+The following `diff` command should not show any output:
+
+```sh
+diff -u <(grep ^JANSSON_OBJ Makefile | head -1 | cut -f 2- -d = | tr ' ' '\n' | sed -Ee 's/\.o$/.c/g' | sort)\
+    <(find src/jansson/src -name '*.c' | sort)
+```
+
+## Unused build files
+
+Autotool build files can be removed, namely:
+- `configure.ac`
+- `jansson.pc.in`
+- `Makefile.am` (2 copies)
+- `jansson_config.h.in`
+
+## Disabled warning
+
+A `#pragma` is used in `src/jansson/src/load.c` to disable overly strict warnings about string truncation.
+No overflow is possible, GCC is only warning about the possibility of a string being truncated if it doesn't fit in the destination buffer.

src/jansson/src/hashtable_seed.c

@@ -41,6 +41,7 @@
 
 #include "jansson.h"
 
+__attribute__ ((unused))
 static uint32_t buf_to_uint32(char *data) {
     size_t i;
     uint32_t result = 0;

src/jansson/src/jansson_private_config.h

@@ -2,16 +2,31 @@
 /* jansson_private_config.h.in.  Generated from configure.ac by autoheader.  */
 
 /* Define to 1 if gcc's __atomic builtins are available */
-#define HAVE_ATOMIC_BUILTINS 1
+#if defined(__has_include) && __has_include(<stdatomic.h>)
+#  include <stdatomic.h>
+#  define HAVE_ATOMIC_BUILTINS 1
+#else
+#  define HAVE_ATOMIC_BUILTINS 0
+#endif
 
 /* Define to 1 if you have the `close' function. */
 #define HAVE_CLOSE 1
 
 /* Define to 1 if you have the <dlfcn.h> header file. */
 #define HAVE_DLFCN_H 1
 
-/* Define to 1 if you have the <endian.h> header file. */
+/* Locate the <endian.h> header file. */
+#if defined(__has_include) && __has_include(<endian.h>)
 #define HAVE_ENDIAN_H 1
+#else
+#define HAVE_ENDIAN_H 0
+#endif
+#if defined(__has_include) && __has_include(<machine/endian.h>)
+#define HAVE_MACHINE_ENDIAN_H 1
+#else
+#define HAVE_MACHINE_ENDIAN_H 0
+#endif
+
 
 /* Define to 1 if you have the <fcntl.h> header file. */
 #define HAVE_FCNTL_H 1
@@ -65,7 +80,11 @@
 #define HAVE_STRTOLL 1
 
 /* Define to 1 if gcc's __sync builtins are available */
+#ifdef __GNUC__
 #define HAVE_SYNC_BUILTINS 1
+#else
+#define HAVE_SYNC_BUILTINS 0
+#endif
 
 /* Define to 1 if you have the <sys/param.h> header file. */
 #define HAVE_SYS_PARAM_H 1