Modernize cmake build system

Provide VexCL::OpenCL, VexCL::Compute, VexCL::CUDA, VexCL::JIT imported
targets, so that users may just

    add_executable(myprogram myprogram.cpp)
    target_link_libraries(myprogram VexCL::OpenCL)

to build a program using the corresponding VexCL backend.
Also stop polluting global cmake namespace with things like
add_definitions(), include_directories(), etc.

.appveyor.yml

@@ -11,7 +11,7 @@ before_build:
     #- type msiexec.log
 build_script:
     - mkdir build && cd build
-    - cmake -G"Visual Studio 14 Win64" ..
+    - cmake -G"Visual Studio 14 Win64" -DVEXCL_BUILD_EXAMPLES=ON -DVEXCL_BUILD_TESTS=ON ..
     - cmake --build . --config Release
 test_script:
     - ctest --output-on-failure

.travis.yml

@@ -41,7 +41,7 @@ before_install:
     - . .travis/install_boost.sh;
 script:
     - mkdir -p build && cd build
-    - cmake -DTEST_COVERAGE=ON -DVEXCL_BACKEND=${VEXCL_BACKEND} ..
+    - cmake -DTEST_COVERAGE=ON -DVEXCL_BUILD_TESTS=ON -DVEXCL_BUILD_EXAMPLES=ON -DVEXCL_BACKEND=${VEXCL_BACKEND} ..
     - make
     - export CPATH=${CPATH}:${BOOST_ROOT}
     - ./examples/devlist && ctest --output-on-failure