Redo CMake configuration.

The style was changed to match the official manual [1], the install
configuration was simplified and now matches the official packaging
guide [2], and the config files use the CMake-specific variable syntax
${VAR} instead of the autoconf-compatible syntax @VAR@, as documented in
[3]. The public header files are declared as such (for CMake 3.3+), and
the generated headers are included in the library target definition.

The tests are only built if SNAPPY_BUILD_TESTS (default ON) is true, so
zippy can be easily used in projects that add_subdirectory() its source
code directly, instead of using find_package().

[1] https://cmake.org/cmake/help/git-master/manual/cmake-language.7.html
[2] https://cmake.org/cmake/help/git-master/manual/cmake-packages.7.html
[3] https://cmake.org/cmake/help/git-master/command/configure_file.html

.appveyor.yml

@@ -31,11 +31,11 @@ build_script:
   - mkdir out
   - cd out
   - if "%platform%"=="x64" set CMAKE_GENERATOR=%CMAKE_GENERATOR% Win64
+  - cmake --version
   - cmake .. -G "%CMAKE_GENERATOR%"
       -DCMAKE_CONFIGURATION_TYPES="%CONFIGURATION%"
   - cmake --build . --config %CONFIGURATION%
   - cd ..
-  - cmake --version
 
 test_script:
-  - out\%CONFIGURATION%\snappy-unittest
+  - out\%CONFIGURATION%\snappy_unittest