Cmake overhaul (#53)

* remove biicode support

The company/startup died in 2015 and the open source project seems
to be dead as well.

* define compiler warning flags globally instead of locally for every binary

* add support for cmake's find_package() (refs #52)

This rewrite of the main CMakeLists.txt cleans up the way ICU flags
are attached to the library target and adds the cmake helper files
(cxxopts-config.cmake, cxxopts-config-version.cmake,
cxxopts-targets.cmake) which are needed for exporting the cxxopts
target. Cmake's find_package command uses these files when the
library is consumed by another project.

Additionally, two new tests have been added which build the example
application via add_subdirectory and find_package.

* removed target_sources from interface library

Adding target_sources to interface libraries and exporting them is
not supported in CMake 3.1 and 3.2. Furthermore, since it is a header,
it is not needed at all.

* use the *_LDFLAGS instead of *_LIBARIES when linking ICU

The LIBRARIES variable seems to contain only the name and not the
full path.

src/CMakeLists.txt

@@ -21,10 +21,4 @@
 if(CXXOPTS_BUILD_EXAMPLES)
     add_executable(example example.cpp)
     target_link_libraries(example cxxopts)
-
-    if (MSVC)
-        target_compile_options(example PUBLIC /W2)
-    elseif(CMAKE_CXX_COMPILER_ID MATCHES "[Cc]lang" OR CMAKE_CXX_COMPILER_ID MATCHES "GNU")
-        target_compile_options(example PUBLIC -std=c++11 -Wall)
-    endif()
 endif()