export CMake Target

[kuepe-sl]

This allows other build systems to easily locate libjpeg under Windows.

Checklist

• Used a personal fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Reset the build number to 0 (if the version changed)
• Re-rendered with the latest conda-smithy (Use the phrase @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• Ensured the license file is being packaged.

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[kuepe-sl]

@conda-forge-admin, please rerender

[beckermr]

I'm not comfortable merging this one. I don't know cmake at all and don't understand the patch.

Maybe someone from @conda-forge/jpeg can look at it?

[kuepe-sl]

It is unfortunate that me renaming the .patch file also obscures the actual changes.
I thought that renaming the .patch file is necessary, because it now creates 2 files instead of 1.

For better readability, here is the patch in more readable form (without some of the cosmetic changes I also did):

diff -Nru a/CMakeLists.txt b/CMakeLists.txt
--- a/CMakeLists.txt	2022-03-08 20:59:49.202400075 +0100
+++ b/CMakeLists.txt	2022-03-08 21:00:20.642490548 +0100
@@ -5,9 +5,13 @@
 # to use cmake and set the WINDOWS_EXPORT_ALL_SYMBOLS property
 # to create a DLL with all symbols exported and a implib (.lib)
 # that can be linked against.
+# This way we can also easily provide package configs for use
+# with other projects.
 
 cmake_minimum_required (VERSION 3.4)
-project (JPEG)
+project (libjpeg VERSION 9)
+
+include(GNUInstallDirs)
 
 set(JPEG_SRC jaricom.c jcapimin.c jcapistd.c jcarith.c jccoefct.c jccolor.c 
         jcdctmgr.c jchuff.c jcinit.c jcmainct.c jcmarker.c jcmaster.c 
@@ -18,8 +22,11 @@
         jfdctfst.c jfdctint.c jidctflt.c jidctfst.c jidctint.c jquant1.c 
         jquant2.c jutils.c jmemmgr.c jmemansi.c)
 add_library (libjpeg SHARED ${JPEG_SRC})
         
-target_include_directories (libjpeg PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
+target_include_directories (libjpeg PUBLIC
+  $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}>
+  $<INSTALL_INTERFACE:include>)
+
 set_target_properties(libjpeg PROPERTIES WINDOWS_EXPORT_ALL_SYMBOLS TRUE)
 add_definitions(-D_CRT_SECURE_NO_WARNINGS)
 
@@ -36,10 +43,33 @@
 
 add_library(jpeg STATIC ${JPEG_SRC})
 
+set(TARGETS_EXPORT_NAME "${PROJECT_NAME}Targets")
+
 install(TARGETS libjpeg cjpeg djpeg jpegtran rdjpgcom wrjpgcom jpeg
+  EXPORT ${TARGETS_EXPORT_NAME}
   RUNTIME DESTINATION bin
   LIBRARY DESTINATION lib
   ARCHIVE DESTINATION lib)
   
 install(FILES jpeglib.h jconfig.h jmorecfg.h jerror.h jpegint.h DESTINATION include)  
  
+
+include(CMakePackageConfigHelpers)
+
+set(TARGETS_FILENAME "${TARGETS_EXPORT_NAME}.cmake")	# this variable is used by cmake-config.in
+set(JPEG_CONFIG_INSTALL_DIR "lib/cmake/${PROJECT_NAME}")
+set(JPEG_PROJECT_CONFIG "${PROJECT_BINARY_DIR}/${PROJECT_NAME}Config.cmake")
+set(JPEG_VERSION_CONFIG "${PROJECT_BINARY_DIR}/${PROJECT_NAME}ConfigVersion.cmake")
+set(JPEG_TARGETS_CONFIG "${PROJECT_BINARY_DIR}/${TARGETS_FILENAME}")
+configure_package_config_file("${PROJECT_SOURCE_DIR}/cmake-config.in" "${JPEG_PROJECT_CONFIG}"
+  INSTALL_DESTINATION "${JPEG_CONFIG_INSTALL_DIR}")
+write_basic_package_version_file("${JPEG_VERSION_CONFIG}"
+  VERSION ${PROJECT_VERSION}
+  COMPATIBILITY SameMajorVersion)
+export(EXPORT ${TARGETS_EXPORT_NAME}
+  FILE ${JPEG_TARGETS_CONFIG})
+
+install(FILES ${JPEG_PROJECT_CONFIG} ${JPEG_VERSION_CONFIG}
+  DESTINATION ${JPEG_CONFIG_INSTALL_DIR})
+install(EXPORT ${TARGETS_EXPORT_NAME}
+  DESTINATION ${JPEG_CONFIG_INSTALL_DIR})
diff -Nru a/cmake-config.in b/cmake-config.in
--- a/cmake-config.in	1970-01-01 01:00:00.000000000 +0100
+++ b/cmake-config.in	2022-03-08 21:00:20.642490548 +0100
@@ -0,0 +1,4 @@
+@PACKAGE_INIT@
+
+include("${CMAKE_CURRENT_LIST_DIR}/@TARGETS_FILENAME@")
+check_required_components("@PROJECT_NAME@")

All the previously installed files are still installed the same way and to the same paths.
I only additionally install a CMake package config into lib/cmake/libjpeg/.
This allows users to find the package using CMake's find_package function.

[gillins]

I think I was involved in the original development of this patch. We did so since there was no MSVC support in jpeg and using cmake to do this seemed easiest, although I am definitely no expert in using it. @isuruf may have some thoughts?

My main concern with this is that it is only for Windows. Linux and OSX won't be improved by this patch - I think we should try to keep the platforms as similar as possible although I'm happy to be overruled.

[gillins]

@kuepe-sl does #36 address this problem? I think cmake has pkg_config support.

[kuepe-sl]

Sorry for the late reply.

Yes, the pkg-config files solve the problem.
I confirmed that GStreamer's install process can locate the jpeg library using pkg-config.