meson: *-user: only descend into *-user when configured

To increase flexibility, only descend into *-user when that is
configured. This allows *-user to selectively include directories based
on the host OS which may not exist on all hosts. Adopt Paolo's
suggestion of checking the configuration in the directories that know
about the configuration.

Message-Id: <20210926220103.1721355-2-f4bug@amsat.org>
Message-Id: <20210926220103.1721355-3-f4bug@amsat.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Warner Losh <wlosh@bsdimp.com>
Acked-by: Paolo Bonzini <pbonzinni@redhat.com>
Reviewed-by: Kyle Evans <kevans@FreeBSD.org>

bsd-user/meson.build

@@ -1,3 +1,7 @@
+if not have_bsd_user
+   subdir_done()
+endif
+
 bsd_user_ss.add(files(
   'bsdload.c',
   'elfload.c',

linux-user/meson.build

@@ -1,3 +1,7 @@
+if not have_linux_user
+   subdir_done()
+endif
+
 linux_user_ss.add(files(
   'elfload.c',
   'exit.c',

meson.build

@@ -40,12 +40,15 @@ config_host_data = configuration_data()
 genh = []
 
 target_dirs = config_host['TARGET_DIRS'].split()
-have_user = false
+have_linux_user = false
+have_bsd_user = false
 have_system = false
 foreach target : target_dirs
-  have_user = have_user or target.endswith('-user')
+  have_linux_user = have_linux_user or target.endswith('linux-user')
+  have_bsd_user = have_bsd_user or target.endswith('bsd-user')
   have_system = have_system or target.endswith('-softmmu')
 endforeach
+have_user = have_linux_user or have_bsd_user
 have_tools = 'CONFIG_TOOLS' in config_host
 have_block = have_system or have_tools
 
@@ -2595,10 +2598,11 @@ subdir('bsd-user')
 subdir('linux-user')
 subdir('ebpf')
 
-bsd_user_ss.add(files('gdbstub.c'))
+common_ss.add(libbpf)
+
 specific_ss.add_all(when: 'CONFIG_BSD_USER', if_true: bsd_user_ss)
 
-linux_user_ss.add(files('gdbstub.c', 'thunk.c'))
+linux_user_ss.add(files('thunk.c'))
 specific_ss.add_all(when: 'CONFIG_LINUX_USER', if_true: linux_user_ss)
 
 # needed for fuzzing binaries