Check for z_size_t along with {crc,adler}32_z in extconf.rb (#69)

The android NDK (android-ndk-r21e) does not have crc32_z, adler32_z, nor
z_size_t in its zlib.h header file. However, it _does_ have the crc32_z
and adler32_z symbols in the libz.a static library!

mkmf performs two tests for have_func:
* It sees if a program that includes the header and takes the address of
  the symbol can compile
* It sees if a program that defines the symbol as `extern void sym_name()`
  and calls it can be linked

If either test works, it considers the function present. The
android-ndk-r21e is passing the second test but not the first for
crc32_z/adler32_z. So, we define HAVE_ZLIB_SIZE_T_FUNCS, but then can't
actually compile the extension (since the prototypes aren't in the
header file).

We can keep this working how it was working before by _also_ checking
for `have_type("z_size_t", "zlib.h")`. The have_type check _only_ looks
in the header file for the type; if a program including the header file
and using the type can't compile, the type is considered absent
regardless of what might be in libz.a.

Author: KJTsanaktsidis

ext/zlib/extconf.rb

@@ -127,7 +127,9 @@
     have_func('crc32_combine', 'zlib.h')
     have_func('adler32_combine', 'zlib.h')
     have_type('z_crc_t', 'zlib.h')
-    if have_func('crc32_z', 'zlib.h') && have_func('adler32_z', 'zlib.h')
+    if (have_type('z_size_t', 'zlib.h') &&
+        have_func('crc32_z', 'zlib.h') &&
+        have_func('adler32_z', 'zlib.h'))
       $defs << "-DHAVE_ZLIB_SIZE_T_FUNCS"
     end
   end