ext/gettext/config.m4: symlink en_US.UTF-8 test bits to en_US for musl

The gettext() family of functions under musl does not support codeset
suffixes like ".UTF-8", because the only codeset it understands is
UTF-8. (Yes, it is annoying that it doesn't support the suffix for the
codeset that it does understand; no, I am not in charge.) Thanks to
this, we have six failing tests on musl,

  * FAIL Gettext basic test with en_US locale that should be on nearly
    every system
    [ext/gettext/tests/gettext_basic-enus.phpt]

  * FAIL Test if bindtextdomain() returns string id if no directory path
    is set( if directory path is 'null')
    [ext/gettext/tests/gettext_bindtextdomain-cwd.phpt]

  * FAIL Test dcgettext() functionality
    [ext/gettext/tests/gettext_dcgettext.phpt]

  * FAIL Test dgettext() functionality
    [ext/gettext/tests/gettext_dgettext.phpt]

  * FAIL Test if dngettext() returns the correct translations
    (optionally plural).
    [ext/gettext/tests/gettext_dngettext-plural.phpt]

  * FAIL Test ngettext() functionality
    [ext/gettext/tests/gettext_ngettext.phpt]

These are all fixed by symlinking the en_US.UTF-8 message data to en_US,
where musl is able to find it.

This does not make the situation any better for developers (who don't
know what libc their users will be running), but that problem is
inhereted from C and is not the fault of the gettext extension.

This partially addresses GH #13696

Author: orlitzky

.gitignore

@@ -177,6 +177,9 @@ php
 /ext/*/configure.ac
 /ext/*/run-tests.php
 
+# Generated by ./configure if libc might be musl
+/ext/gettext/tests/locale/en_US
+
 # ------------------------------------------------------------------------------
 # Generated by Windows build system
 # ------------------------------------------------------------------------------

ext/gettext/config.m4

@@ -25,6 +25,17 @@ if test "$PHP_GETTEXT" != "no"; then
     AC_CHECK_LIB(c, bindtextdomain, [
       GETTEXT_LIBS=
       GETTEXT_CHECK_IN_LIB=c
+
+      dnl If libintl.h is provided by libc, it's possible that libc is musl.
+      dnl The gettext family of functions under musl ignores the codeset
+      dnl suffix on directories like "en_US.UTF-8"; instead they look only
+      dnl in "en_US". To accomodate that, we symlink some test data from one
+      dnl to the other.
+      AC_MSG_NOTICE([symlinking en_US.UTF-8 messages to en_US in case you are on musl])
+      _linkdest="${srcdir%/}"/ext/gettext/tests/locale/en_US
+      AS_IF([test ! -e "${_linkdest}"],[
+        ln -s en_US.UTF-8 "${_linkdest}"
+      ])
     ],[
       AC_MSG_ERROR(Unable to find required gettext library)
     ])