Expose headers in cmake project (#77)

Expose through CMake directives, the name of the project so that package managers such as
CPM (https://github.com/cpm-cmake/CPM.cmake) could consume this library cleanly

Author: damageboy

CMakeLists.txt

@@ -9,4 +9,7 @@ if(BUILD_TESTING)
     add_subdirectory(test)
 endif()
 
+add_library(subprocess INTERFACE)
+target_include_directories(subprocess INTERFACE .)
+
 install(FILES subprocess.hpp DESTINATION include/cpp-subprocess/)