php74.extensions.iconv: fix error signalling

The configure script checks whether iconv supports errno. Unfortunately, on PHP < 8, the test program includes $PHP_ICONV_H_PATH, which defaults to FHS path so it fails to build:

conftest.c:13:10: fatal error: /usr/include/iconv.h: No such file or directory
13 | #include </usr/include/iconv.h>
| ^~~~~~~~~~~~~~~~~~~~~~

That causes the feature check to report a false negative, leading PHP to use a degraded code that returns PHP_ICONV_ERR_UNKNOWN when error occurs, breaking granular error handling in applications.

To prevent this, let’s just include <iconv.h>.

PHP 8 just uses include path so the detection works there: https://github.com/php/php-src/commit/7bd1d703411e1e4b1f663f0cb06af619eea04b42

(cherry picked from commit 024243bac4612c62ef5be676818ed2465edae27f)

pkgs/top-level/php-packages.nix

···
         configureFlags = [ "--with-gmp=${gmp.dev}" ]; }
       { name = "hash"; enable = lib.versionOlder php.version "7.4"; }
       { name = "iconv";
-        configureFlags = if stdenv.isDarwin then
-                           [ "--with-iconv=${libiconv}" ]
-                         else
-                           [ "--with-iconv" ];
+        configureFlags = [
+          "--with-iconv${lib.optionalString stdenv.isDarwin "=${libiconv}"}"
+        ];
+        patches = lib.optionals (lib.versionOlder php.version "8.0") [
+          # Header path defaults to FHS location, preventing the configure script from detecting errno support.
+          (fetchpatch {
+            url = "https://github.com/fossar/nix-phps/raw/263861a8c9bdafd7abe44db6db4ef0179643680c/pkgs/iconv-header-path.patch";
+            sha256 = "7GHnEUu+hcsQ4h3itDwk6p46ZKfib9JZ2XpWlXrdn6E=";
+          })
+        ];
         doCheck = false; }
       { name = "imap";
         buildInputs = [ uwimap openssl pam pcre' ];