pngminus: Update CMake file; rename test scripts; add .gitignore

Raise the minimum required CMake version to 3.5.

Add the configuration option `PNGMINUS_USE_SYSTEM_PNG` for compiling
and linking with the system libpng library instead of the internal one.

Remove the old configuration option `PNGMINUS_USE_STATIC_LIBRARIES`.
When using the internal libpng (via `PNGMINUS_USE_SYSTEM_PNG=OFF`),
simply enforce static linking and produce single-file executables.

Rename the scripts "png2pnm.sh" (etc.) to "test_png2pnm.sh" (etc.),
to make it obvious that they are test drivers, not program launchers.

Add a .gitignore file for project-specific build and test artifacts.

contrib/pngminus/.gitignore

@@ -0,0 +1,11 @@
+# Compiled executables
+png2pnm
+png2pnm.exe
+png2pnm-static*
+pnm2png
+pnm2png.exe
+pnm2png-static*
+
+# Test artifacts
+*.png
+*.p[abgnp]m

contrib/pngminus/CMakeLists.txt

@@ -1,24 +1,41 @@
-cmake_minimum_required(VERSION 3.1)
-cmake_policy(VERSION 3.1)
+# Copyright (c) 2018-2024 Cosmin Truta
+#
+# This software is released under the MIT license. For conditions of
+# distribution and use, see the LICENSE file part of this package.
+
+cmake_minimum_required(VERSION 3.5)
 
 project(PNGMINUS C)
 
-option(PNGMINUS_USE_STATIC_LIBRARIES "Use the static library builds" ON)
+option(PNGMINUS_USE_SYSTEM_PNG
+       "Use the libpng build found in the system" OFF)
 
-# libpng
-add_subdirectory(../.. libpng)
-include_directories(${CMAKE_CURRENT_SOURCE_DIR}/../..)
-include_directories(${CMAKE_CURRENT_BINARY_DIR}/libpng)
-if(PNGMINUS_USE_STATIC_LIBRARIES)
-  set(PNGMINUS_PNG_LIBRARY png_static)
-else()
-  set(PNGMINUS_PNG_LIBRARY png)
-endif()
-
-# png2pnm
 add_executable(png2pnm png2pnm.c)
-target_link_libraries(png2pnm ${PNGMINUS_PNG_LIBRARY})
-
-# pnm2png
 add_executable(pnm2png pnm2png.c)
-target_link_libraries(pnm2png ${PNGMINUS_PNG_LIBRARY})
+
+if(PNGMINUS_USE_SYSTEM_PNG)
+    # Use the system libpng.
+    find_package(PNG REQUIRED)
+    target_link_libraries(png2pnm PRIVATE PNG::PNG)
+    target_link_libraries(pnm2png PRIVATE PNG::PNG)
+else()
+    # Build and use the internal libpng.
+    # Configure libpng for static linking, to produce single-file executables.
+    set(PNG_STATIC ON
+        CACHE STRING "Build the internal libpng as a static library" FORCE)
+    set(PNG_SHARED OFF
+        CACHE STRING "Build the internal libpng as a shared library" FORCE)
+    set(PNG_FRAMEWORK OFF
+        CACHE STRING "Build the internal libpng as a framework bundle" FORCE)
+    add_subdirectory(../.. libpng)
+    target_include_directories(png2pnm PRIVATE
+        "$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/../..>"
+        "$<BUILD_INTERFACE:${CMAKE_CURRENT_BINARY_DIR}/libpng>"
+    )
+    target_include_directories(pnm2png PRIVATE
+        "$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/../..>"
+        "$<BUILD_INTERFACE:${CMAKE_CURRENT_BINARY_DIR}/libpng>"
+    )
+    target_link_libraries(png2pnm PRIVATE png_static)
+    target_link_libraries(pnm2png PRIVATE png_static)
+endif()