spatialite-tools required to build, run tests, but missing from CMake check?

I ran into build problem with tests. We may be missing a dependency check for [spatialite-tools](https://www.gaia-gis.it/fossil/spatialite-tools/index) on a fresh build, but don't explicitly check for its existence when building the test target. As a result, the test target fails downstream when a require data dependency is missing.  



Here's the error I got: 



```bash

$ mkdir build

$ cmake -DENABLE_PYTHON_BINDINGS=OFF \

  -DENABLE_NODE_BINDINGS=OFF  \

  -DCMAKE_BUILD_TYPE=Debug \

  -DENABLE_SANITIZER=ON ../

$ make -j4 tests

...

[ 98%] Building tz.sqlite...

make[3]: *** [test/data/tz.sqlite] Error 1

make[3]: *** Deleting file `test/data/tz.sqlite'

make[2]: *** [test/CMakeFiles/utrecht_tiles.dir/all] Error 2

make[2]: *** Waiting for unfinished jobs....

[ 98%] Built target pedestriancost

[ 98%] Built target isochrone

[ 98%] Built target matrix

make[1]: *** [test/CMakeFiles/tests.dir/rule] Error 2

make: *** [tests] Error 2

```



I fixed this by running `brew install spatialite-tools`. 



Expected behavior: 

- cmake build of test target (via `-DENABLE_TESTS=ON`) checks for spatialite-tools dependency on host, sets bit if present.

- If building tests, check for spatialite-tools dependency bit and early exit if it's missing.

- If tests are off (via `-DENABLE_TESTS=OFF`), don't load or check for dependency.



My system: 



```

$ sw_vers

ProductName:	Mac OS X

ProductVersion:	10.14.3

BuildVersion:	18D109



$ gcc --version

Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/usr/include/c++/4.2.1

Apple LLVM version 10.0.0 (clang-1000.10.44.4)

Target: x86_64-apple-darwin18.2.0

Thread model: posix

InstalledDir: /Library/Developer/CommandLineTools/usr/bin

```

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