doctest implementation as static library

[ymadzhunkov]

Add cmake target - doctest implementation as static library

I am using doctest in several projects ( at home and at work ). As number of tests grow, compile time grows, because doctest implementation is compiled multiple times. In order to save compile time, I created a static library and linked it in all my tests executables. I want to move build of this static library in doctest repo, so I can reuse it all my projects.

My workflow is as follows (doctest is submodule in my git repo)
In my main CMakeLists.txt

SET(DOCTEST_WITH_TESTS OFF CACHE BOOL "Build tests/examples" FORCE)
add_subdirectory(doctest)

Adding Unit test whenever is needed

add_executable(${UNIT_TEST} ${ARG_TEST_SOURCES} )
target_link_libraries(${UNIT_TEST} doctest_with_main)