CMake options:
* DISCOUNT_MAKE_INSTALL - Set to OFF to disable install rules (default is ON);
* DISCOUNT_INSTALL_SAMPLES - Set to ON to install sample programs (default is OFF).
Export `discount` package with targets:
* discount::libmarkdown;
* discount::markdown;
* discount::mkd2html (optional);
* discount::makepage (optional).
Usage from CMake project:
find_package(discount)
add_executable(program main.c)
target_link_libraries(program discount::libmarkdown)