https://bugs.gentoo.org/900152

It doesn't make sense to default XOPEN_SOURCE and subsequently DEFAULT_SOURCE
as DEFAULT_SOURCE is asking for no extensions. Anyway, just ask for GNU_SOURCE
as it'll placate both musl and glibc for all of strptime/timegm/getpass and
gcr-3 is obsolete anyway.
--- a/gck/meson.build
+++ b/gck/meson.build
@@ -77,8 +77,7 @@ gck_cflags = [
   '-DGCK_API_SUBJECT_TO_CHANGE',
   '-DP11_KIT_API_SUBJECT_TO_CHANGE',
   '-DPKCS11_REGISTRY_DIR="@0@"'.format(get_option('prefix') / get_option('libdir') / 'pkcs11'),
-  '-D_XOPEN_SOURCE',   # Needed for strptime()
-  '-D_DEFAULT_SOURCE', # Needed for timegm
+  '-D_GNU_SOURCE',   # Needed for strptime(), timegm, getpass
 ]
 
 gck_symbolmap = meson.current_source_dir() / 'libgck.map'
--- a/gcr/meson.build
+++ b/gcr/meson.build
@@ -146,7 +146,7 @@ gcr_base_cflags = [
   '-DGCR_API_SUBJECT_TO_CHANGE',
   '-DGCK_API_SUBJECT_TO_CHANGE',
   '-DP11_KIT_API_SUBJECT_TO_CHANGE',
-  '-D_XOPEN_SOURCE',   # Needed for strptime()
+  '-D_GNU_SOURCE',   # Needed for strptime(), timegm, getpass
 ]
 
 gcr_base_symbolmap = meson.current_source_dir() / 'libgcr-base.map'