howard-hinnant-date: allow access to zoneinfo

This fixes the situtation where, if `/usr/share/zoneinfo` was
inaccessible/didn't otherwise exist, `howard-hinnant-date` would
download and drop a `~/Downloads/tzdata` directory containing some
timezone information from IANA [1]. To avoid this, we make use of the
`tzdata`'s `zoneinfo`, preventing the dropping of random directories and
files.

[1] https://data.iana.org/time-zones/releases/tzdata2019c.tar.gz

pkgs/development/libraries/howard-hinnant-date/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchFromGitHub, cmake, curl, fetchpatch }:
+{ stdenv, fetchFromGitHub, cmake, curl, tzdata, fetchpatch, substituteAll }:
 
 stdenv.mkDerivation rec {
   pname = "howard-hinnant-date-unstable";
@@ -16,6 +16,13 @@ stdenv.mkDerivation rec {
       url = "https://github.com/HowardHinnant/date/commit/e56b2dce7e89a92e1b9b35caa13b3e938c4cedea.patch";
       sha256 = "0m3qbhq7kmm9qa3jm6d2px7c1dxdj5k9lffgdvqnrwmhxwj1p9n2";
     })
+    # Without this patch, this library will drop a `tzdata` directory into
+    # `~/Downloads` if it cannot find `/usr/share/zoneinfo`. Make the path it
+    # searches for `zoneinfo` be the one from the `tzdata` package.
+    (substituteAll {
+      src = ./make-zoneinfo-available.diff;
+      inherit tzdata;
+    })
   ];
 
   nativeBuildInputs = [ cmake ];
@@ -24,6 +31,7 @@ stdenv.mkDerivation rec {
   cmakeFlags = [
     "-DBUILD_TZ_LIB=true"
     "-DBUILD_SHARED_LIBS=true"
+    "-DUSE_SYSTEM_TZ_DB=true"
   ];
 
   outputs = [ "out" "dev" ];

pkgs/development/libraries/howard-hinnant-date/make-zoneinfo-available.diff

@@ -0,0 +1,13 @@
+diff --git a/src/tz.cpp b/src/tz.cpp
+index 68436c3..2bfe19e 100644
+--- a/src/tz.cpp
++++ b/src/tz.cpp
+@@ -349,7 +349,7 @@ discover_tz_dir()
+     struct stat sb;
+     using namespace std;
+ #  ifndef __APPLE__
+-    CONSTDATA auto tz_dir_default = "/usr/share/zoneinfo";
++    CONSTDATA auto tz_dir_default = "@tzdata@/share/zoneinfo";
+     CONSTDATA auto tz_dir_buildroot = "/usr/share/zoneinfo/uclibc";
+ 
+     // Check special path which is valid for buildroot with uclibc builds