Fix CMake warning about mis-matching arguments

[bostick]

When running the zlib tests, I get this CMake warning:

8: CMake Warning (dev) in CMakeLists.txt:
8:   A logical block opening on the line
8: 
8:     /Users/brenton/development/github/tbt-parser/build/_deps/zlib-build/test/zlib_add_subdirectory_test/CMakeLists.txt:23 (if)
8: 
8:   closes on the line
8: 
8:     /Users/brenton/development/github/tbt-parser/build/_deps/zlib-build/test/zlib_add_subdirectory_test/CMakeLists.txt:27 (endif)
8: 
8:   with mis-matching arguments.
8: This warning is for project developers.  Use -Wno-dev to suppress it.

This is because zlib is doing:

if(ZLIB_BUILD_STATIC)
...
endif(@ZLIB_BUILD_STATIC)

instead of doing:

if(ZLIB_BUILD_STATIC)
...
endif(ZLIB_BUILD_STATIC)