meson: define SIZEOF_LONG and use -Wundef

meson builds defaulted to SIZEOF_LONG=0 in various places

meson.build

@@ -36,6 +36,7 @@ add_project_arguments(
     '-Wdeclaration-after-statement',
     '-fno-strict-aliasing',
     '-fvisibility=hidden',
+    '-Wundef',
   ]),
   language : ['c']
 )
@@ -445,6 +446,8 @@ if host_machine.endian() == 'big'
   config.set('WORDS_BIGENDIAN', 1)
 endif
 
+config.set('SIZEOF_LONG', cc.sizeof('long'))
+
 # Required to make pixman-private.h
 config.set('PACKAGE', 'foo')