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

.travis.yml

@@ -47,13 +47,13 @@ install:
 - echo ${CC}
 - echo ${CXX}
 - ${CXX} --version
+- cmake --version
 
 before_script:
 - mkdir -p build && cd build
 - cmake .. -DCMAKE_BUILD_TYPE=$BUILD_TYPE
 - cmake --build .
 - cd ..
-- cmake --version
 
 script:
-- build/snappy-unittest
+- build/snappy_unittest