test: fix "get define" on OpenBSD

Same as with NetBSD, OpenBSD doesn't provide a ZLIB_VER_MAJOR but has
ZLIB_VERSION instead.

test cases/common/137 get define/meson.build

@@ -32,6 +32,9 @@ foreach lang : ['c', 'cpp']
   elif host_system == 'netbsd'
     d = cc.get_define('__NetBSD__')
     assert(d == '1', '__NetBSD__ value is @0@ instead of 1'.format(d))
+  elif host_system == 'openbsd'
+    d = cc.get_define('__OpenBSD__')
+    assert(d == '1', '__OpenBSD__ value is @0@ instead of 1'.format(d))
   elif host_system == 'gnu'
     d = cc.get_define('__GNU__')
     assert(d == '1', '__GNU__ value is @0@ instead of 1'.format(d))
@@ -44,10 +47,10 @@ foreach lang : ['c', 'cpp']
     # found in the compiler's default search path, GCC inserts an extra comment
     # between the delimiter and the define which causes a parsing error.
     # https://github.com/mesonbuild/meson/issues/1726
-    if host_machine.system() == 'netbsd'
-      # NetBSD's zlib doesn't is version 1.2.3 and doesn't have a
-      # ZLIB_VER_MAJOR, but it does have a ZLIB_VERSION (which is a string), so
-      # check the first non-quote character of that.
+    if host_machine.system() == 'netbsd' or host_machine.system() == 'openbsd'
+      # NetBSD and OpenBSD's zlib don't have a ZLIB_VER_MAJOR, but they do have
+      # a ZLIB_VERSION (which is a string), so check the first non-quote
+      # character of that.
       ver = cc.get_define('ZLIB_VERSION', prefix : '#include <zlib.h>')[1]
       assert(ver == '1', 'ZLIB_VERSION (major) value is "@0@" instead of "1"'.format(ver))
     else