musl: meson: gracefully disable gshadow, nss, and idn support

- musl does not support gshadow, and does not provide gshadow.h,
- musl does not support nss, and does not provide nss.h which is necessary
  for each nss modules,
- musl does not provide NI_IDN.

meson.build

@@ -708,6 +708,8 @@ foreach header : [
 endforeach
 
 foreach header : [
+        'gshadow.h',
+        'nss.h',
         'sys/sdt.h',
         'threads.h',
         'valgrind/memcheck.h',
@@ -718,6 +720,20 @@ foreach header : [
                    cc.has_header(header))
 endforeach
 
+foreach ident : [
+                ['NI_IDN', 'netdb.h']
+        ]
+
+        if meson.version().version_compare('>=1.3.0')
+                have = cc.has_define(ident[0],
+                                     prefix : '''#include <@0@>'''.format(ident[1]),
+                                     args : '-D_GNU_SOURCE')
+        else
+                have = cc.has_header_symbol(ident[1], ident[0])
+        endif
+        conf.set10('HAVE_' + ident[0], have)
+endforeach
+
 #####################################################################
 
 fallback_hostname = get_option('fallback-hostname')
@@ -1627,52 +1643,60 @@ conf.set10('ENABLE_NSPAWN', feature.allowed())
 
 conf.set10('DEFAULT_MOUNTFSD_TRUSTED_DIRECTORIES', get_option('default-mountfsd-trusted-directories'))
 
-foreach term : ['analyze',
-                'backlight',
-                'binfmt',
-                'compat-mutable-uid-boundaries',
-                'coredump',
-                'efi',
-                'environment-d',
-                'firstboot',
-                'gshadow',
-                'hibernate',
-                'hostnamed',
-                'hwdb',
-                'idn',
-                'ima',
-                'ipe',
-                'initrd',
-                'kernel-install',
-                'ldconfig',
-                'localed',
-                'logind',
-                'machined',
-                'mountfsd',
-                'networkd',
-                'nsresourced',
-                'nss-myhostname',
-                'nss-systemd',
-                'oomd',
-                'portabled',
-                'pstore',
-                'quotacheck',
-                'randomseed',
-                'resolve',
-                'rfkill',
-                'smack',
-                'sysext',
-                'sysusers',
-                'timedated',
-                'timesyncd',
-                'tmpfiles',
-                'tpm',
-                'userdb',
-                'utmp',
-                'vconsole',
-                'xdg-autostart']
-        have = get_option(term)
-        name = 'ENABLE_' + term.underscorify().to_upper()
+foreach tuple : [
+                ['analyze'],
+                ['backlight'],
+                ['binfmt'],
+                ['compat-mutable-uid-boundaries'],
+                ['coredump'],
+                ['efi'],
+                ['environment-d'],
+                ['firstboot'],
+                ['gshadow',         conf.get('HAVE_GSHADOW_H') == 1,  'gshadow.h not found'],
+                ['hibernate'],
+                ['hostnamed'],
+                ['hwdb'],
+                ['idn',             conf.get('HAVE_NI_IDN') == 1,     'NI_IDN is not defined'],
+                ['ima'],
+                ['ipe'],
+                ['initrd'],
+                ['kernel-install'],
+                ['ldconfig'],
+                ['localed'],
+                ['logind'],
+                ['machined'],
+                ['mountfsd'],
+                ['networkd'],
+                ['nsresourced'],
+                ['nss-myhostname',  conf.get('HAVE_NSS_H') == 1,      'nss.h not found'],
+                ['nss-systemd',     conf.get('HAVE_NSS_H') == 1,      'nss.h not found'],
+                ['oomd'],
+                ['portabled'],
+                ['pstore'],
+                ['quotacheck'],
+                ['randomseed'],
+                ['resolve'],
+                ['rfkill'],
+                ['smack'],
+                ['sysext'],
+                ['sysusers'],
+                ['timedated'],
+                ['timesyncd'],
+                ['tmpfiles'],
+                ['tpm'],
+                ['utmp'],
+                ['userdb'],
+                ['vconsole'],
+                ['xdg-autostart'],
+        ]
+
+        have = get_option(tuple[0])
+        if have and tuple.length() >= 3 and not tuple[1]
+                warning('@0@ support is requested but @1@, disabling it'.format(tuple[0], tuple[2]))
+                have = false
+        endif
+
+        name = 'ENABLE_' + tuple[0].underscorify().to_upper()
         conf.set10(name, have)
 endforeach
 
@@ -1681,11 +1705,16 @@ enable_sysusers = conf.get('ENABLE_SYSUSERS') == 1
 foreach tuple : [['nss-mymachines', 'machined'],
                  ['nss-resolve',    'resolve']]
         want = get_option(tuple[0])
-        if want.allowed()
-                have = get_option(tuple[1])
-                if want.enabled() and not have
+        if want.enabled()
+                if conf.get('HAVE_NSS_H') != 1
+                        error('@0@ is requested but nss.h not found'.format(tuple[0]))
+                endif
+                if not get_option(tuple[1])
                         error('@0@ is requested but @1@ is disabled'.format(tuple[0], tuple[1]))
                 endif
+                have = true
+        elif want.allowed()
+                have = get_option(tuple[1]) and conf.get('HAVE_NSS_H') == 1
         else
                 have = false
         endif