Fix CMake build

Fix CMake build

`MAN_INSTALL_DIR` was undefined.

Tested with `make cmakebuild`, and saw that the man pages got installed to the right place:

```
-- Installing: /Users/terrelln/install_test_dir/lib/pkgconfig/libzstd.pc
-- Installing: /Users/terrelln/install_test_dir/include/zstd.h
-- Installing: /Users/terrelln/install_test_dir/include/zbuff.h
-- Installing: /Users/terrelln/install_test_dir/include/zdict.h
-- Installing: /Users/terrelln/install_test_dir/include/zstd_errors.h
-- Installing: /Users/terrelln/install_test_dir/lib/libzstd.1.3.4.dylib
-- Installing: /Users/terrelln/install_test_dir/lib/libzstd.dylib
-- Installing: /Users/terrelln/install_test_dir/lib/libzstd.a
-- Installing: /Users/terrelln/install_test_dir/bin/zstd
-- Up-to-date: /Users/terrelln/install_test_dir/bin/zstd
-- Installing: /Users/terrelln/install_test_dir/bin/zstdcat
-- Installing: /Users/terrelln/install_test_dir/bin/unzstd
-- Installing: /Users/terrelln/install_test_dir/share/man/man1/zstd.1
-- Installing: /Users/terrelln/install_test_dir/share/man/man1/zstdcat.1
-- Installing: /Users/terrelln/install_test_dir/share/man/man1/unzstd.1
-- Installing: /Users/terrelln/install_test_dir/bin/zstdmt
-- Installing: /Users/terrelln/install_test_dir/bin/pzstd
-- Installing: /Users/terrelln/install_test_dir/zstd_manual.html
```