provide `Findpigpio.cmake`

[rursprung]

currently there's no easy way to automate the integration of pigpio into other cmake projects.

using ExternalProject i found a way to do it. for this i created a cmake/Findpigpio.cmake in my project with the following content:

include(ExternalProject)
ExternalProject_Add(pigpio
  GIT_REPOSITORY    https://github.com/joan2937/pigpio.git
  GIT_TAG           v79
  CMAKE_ARGS        -DCMAKE_INSTALL_PREFIX:PATH=<INSTALL_DIR>
)

ExternalProject_Get_Property(pigpio INSTALL_DIR)
set(pigpio_INCLUDE_DIR ${INSTALL_DIR}/include)
set(pigpio_LIBRARIES ${INSTALL_DIR}/lib/libpigpio.so)

mark_as_advanced(pigpio_INCLUDE_DIR pigpio_LIBRARIES)

afterwards i could use this in my main CMakeLists.txt:

list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_LIST_DIR}/cmake)
# [..]
find_package(pigpio REQUIRED)
# [..]
include_directories(
  # [..]
  ${pigpio_INCLUDE_DIR}
)
# [..]
add_dependencies(${PROJECT_NAME} pigpio)
# [..]
target_link_libraries(
  ${PROJECT_NAME} 
  # [..]
  ${pigpio_LIBRARIES}
)

i think it'd be great if this project could ship something like this (i'm not sure if this is the best/proper way of doing it) so that others can pull it in more easily.