Add cmake support with cmake-config file

Add full cmake support. The project can either be used with
`add_subdirectory` or be installed into the system (or some other
directory) and be found with `find_package(lodepng)`. In both cases the
cmake target `lodepng::lodepng` is all that needs to be linked.

Having an install target also makes packaging easier.

- add libraries
  - loadepng
  - lodepng_util
- add executables (also installed)
  - pngdetail
- generate lodepng-config.cmake and install in lib/cmake/lodepng
- add unittest and target `test` (enabled by flag ENABLE_TESTING)
- add benchmark (enabled by flag BUILD_BENCHMARK)
- add examples (enabled by flag BUILD_EXAMPLES)

note: I was only able to install SDL2 from my package manager, but
benchmark, example_opengl and example_sdl use SDL (1)

CMakeLists.txt

@@ -0,0 +1,104 @@
+cmake_minimum_required(VERSION 3.1)
+project(lodepng)
+
+# define target to install and link tests against
+add_library(${CMAKE_PROJECT_NAME}
+  lodepng.h
+  lodepng.cpp)
+target_include_directories(${CMAKE_PROJECT_NAME}
+  PUBLIC $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>)
+add_library(${CMAKE_PROJECT_NAME}_util
+  lodepng_util.h
+  lodepng_util.cpp)
+target_link_libraries(${CMAKE_PROJECT_NAME}_util PUBLIC ${CMAKE_PROJECT_NAME})
+add_executable(pngdetail
+  pngdetail.cpp)
+target_link_libraries(pngdetail PRIVATE ${CMAKE_PROJECT_NAME}_util)
+
+# ALIAS same as in configure file
+# the ALIAS can be used to create examples which use the same syntax as a client
+# application, which uses `find_package(lodepng CONFIG)`
+# create the alias lodepng::lodepng
+add_library(${CMAKE_PROJECT_NAME}::${CMAKE_PROJECT_NAME} ALIAS ${CMAKE_PROJECT_NAME})
+add_library(${CMAKE_PROJECT_NAME}::${CMAKE_PROJECT_NAME}_util ALIAS ${CMAKE_PROJECT_NAME}_util)
+
+option(ENABLE_TESTING "enable creation of unittest" ON)
+if(ENABLE_TESTING)
+  enable_testing()
+  add_executable(${CMAKE_PROJECT_NAME}_unittest lodepng_unittest.cpp)
+  # add compiler flags for test target only for GCC and Clang
+  target_compile_options(${CMAKE_PROJECT_NAME}_unittest PRIVATE
+    $<BUILD_INTERFACE:
+      $<$<OR:$<CXX_COMPILER_ID:GNU>,$<CXX_COMPILER_ID:Clang>>:
+        -Werror -Wall -pedantic>>
+    )
+
+  add_test(test_${CMAKE_PROJECT_NAME}_unittest ${CMAKE_PROJECT_NAME}_unittest)
+  target_link_libraries(${CMAKE_PROJECT_NAME}_unittest PRIVATE ${CMAKE_PROJECT_NAME}_util)
+endif()
+
+option(BUILD_EXAMPLES "build examples using ${CMAKE_PROJECT_NAME}" OFF)
+if(BUILD_EXAMPLES)
+  add_subdirectory(examples)
+endif()
+
+option(BUILD_BENCHMARK "build benchmark for ${CMAKE_PROJECT_NAME}, requires SDL2" OFF)
+if(BUILD_BENCHMARK)
+  add_executable(${CMAKE_PROJECT_NAME}_benchmark lodepng_benchmark.cpp)
+  target_link_libraries(${CMAKE_PROJECT_NAME}_benchmark PRIVATE ${CMAKE_PROJECT_NAME}::${CMAKE_PROJECT_NAME})
+  # SDL2 Dependency
+  find_package(SDL2 CONFIG REQUIRED)
+  if (TARGET SDL2::SDL2)
+    message(STATUS "using TARGET SDL2::SDL2")
+    target_link_libraries(${CMAKE_PROJECT_NAME}_benchmark PUBLIC SDL2::SDL2)
+  else()
+    message(STATUS "no TARGET SDL2::SDL2, or SDL2, using variables")
+    target_include_directories(${CMAKE_PROJECT_NAME}_benchmark PUBLIC $<BUILD_INTERFACE:${SDL2_INCLUDE_DIRS}>)
+    target_link_libraries(${CMAKE_PROJECT_NAME}_benchmark PUBLIC ${SDL2_LIBRARIES})
+  endif()
+  add_custom_target(benchmark COMMAND ${CMAKE_PROJECT_NAME}_benchmark)
+endif()
+
+# create install target
+include(GNUInstallDirs)
+install(
+  TARGETS
+  ${CMAKE_PROJECT_NAME}
+  ${CMAKE_PROJECT_NAME}_util
+  pngdetail
+  EXPORT ${CMAKE_PROJECT_NAME}-targets
+  INCLUDES DESTINATION ${CMAKE_INSTALL_INCLUDEDIR} # set include path for installed library target
+  RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
+  LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
+  ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
+  )
+install(
+  FILES
+  lodepng.h
+  lodepng_util.h
+  DESTINATION include
+  )
+
+# Include module for fuctions
+# - 'write_basic_package_version_file'
+# - 'configure_package_config_file'
+include(CMakePackageConfigHelpers)
+
+# generate and install termcolor-config.cmake file
+# Configure '<PROJECT-NAME>-config.cmake'
+configure_package_config_file(
+  "cmake/config.cmake.in"
+  "${CMAKE_CURRENT_BINARY_DIR}/generated/lodepng-config.cmake"
+  INSTALL_DESTINATION "lib/cmake/lodepng"
+)
+# install config file
+install(
+  FILES "${CMAKE_CURRENT_BINARY_DIR}/generated/lodepng-config.cmake"
+  DESTINATION "lib/cmake/lodepng"
+)
+# install targets file
+install(
+  EXPORT "${CMAKE_PROJECT_NAME}-targets"
+  NAMESPACE "${CMAKE_PROJECT_NAME}::"
+  DESTINATION "lib/cmake/lodepng"
+)

cmake/config.cmake.in

@@ -0,0 +1,5 @@
+@PACKAGE_INIT@
+
+include("${CMAKE_CURRENT_LIST_DIR}/@[email protected]")
+check_required_components("@CMAKE_PROJECT_NAME@")
+

examples/CMakeLists.txt

@@ -0,0 +1,37 @@
+
+foreach(example
+    4bit_palette
+    bmp2png
+    decode
+    encode
+    gzip
+    #opengl
+    optimize_png
+    png2bmp
+    png_info
+    reencode
+    #sdl
+  )
+  add_executable(example_${example}
+    example_${example}.cpp)
+  target_link_libraries(example_${example} PRIVATE lodepng)
+endforeach()
+
+# SDL2 Dependency
+find_package(SDL2 CONFIG REQUIRED)
+if (TARGET SDL2::SDL2)
+  message(STATUS "using TARGET SDL2::SDL2")
+else()
+  message(STATUS "no TARGET SDL2::SDL2, or SDL2, using variables")
+endif()
+foreach(example
+    #opengl
+    #sdl
+  )
+  if (TARGET SDL2::SDL2)
+    target_link_libraries(example_${example} PRIVATE SDL2::SDL2)
+  else()
+    target_include_directories(example_${example} PRIVATE $<BUILD_INTERFACE:${SDL2_INCLUDE_DIRS}>)
+    target_link_libraries(example_${example} PRIVATE ${SDL2_LIBRARIES})
+  endif()
+endforeach()