[build] Remove a level of quoting from CAIRO_PARSE_VERSION.

The m4 that comes with OpenBSD gets confused by the extra quoting
applied to the version number argument when building the
bodies of the cairo_version_{major,minor,micro} macros using
m4_bpatsubst(). Since we're making sure that the argument is always
a number, we can safely use the unquoted \2 body rather than \[\2\].

acinclude.m4

@@ -38,7 +38,7 @@ m4_define([CAIRO_PARSE_VERSION],dnl
 		m4_translit(dnl
 		m4_bpatsubst(m4_include(cairo-version.h),
 			     [^.define \([a-zA-Z0-9_]*\)  *\([0-9][0-9]*\)],
-			     [[m4_define(\1, \[\2\])]]),
+			     [[m4_define(\1, \2)]]),
 			    [A-Z], [a-z])dnl
 )dnl