meson: Make it possible to build for 32-bit targets

Setting glx_align64 to '' gives a null string in the arguments list passed
to the compiler.  This is taken as an input filename, leading to:

"cc: error: : No such file or directory"

Instead, assign an empty list to glx_align64, which gets flattened to
nothing in the arguments list.

Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>

include/meson.build

@@ -31,7 +31,7 @@ else
     conf_data.set('X_BYTE_ORDER', 'X_BIG_ENDIAN')
 endif
 
-glx_align64 = ''
+glx_align64 = []
 if cc.sizeof('unsigned long') == 8
    conf_data.set('_XSERVER64', '1')
    glx_align64 = '-D__GLX_ALIGN64'