Dependency wrangling part 0/N: standard CMake modules (#7658)

* Hoist Threads::Threads to the top level

* Remove global OpenGL dependency

This is added by the helpers as-needed. Removing it here lets one build
just libHalide without searching for OpenGL.

* Narrow scope of OpenMP to tutorial

Only the tutorial targets actually use OpenMP. Don't search for OpenMP
if WITH_TUTORIALS is off.

* Move JPEG and PNG deps to tools

Only the Halide::ImageIO library uses these directly, so limiting the
scope protects against unintented use.

* Work around CMake bug

The CMake $<TARGET_NAME_IF_EXISTS:...> genex uses dynamic scoping w.r.t.
the target environment, rather than the usual static scoping. This means
we need to move the PNG and JPEG dependencies higher up.

* Add link to CMake issue in comments.

CMakeLists.txt

@@ -91,8 +91,27 @@ endif()
 # Import dependencies
 ##
 
+## Threads
+option(THREADS_PREFER_PTHREAD_FLAG "When enabled, prefer to use the -pthread flag to explicit linking" ON)
+find_package(Threads REQUIRED)
+
+## Complex dependencies
 add_subdirectory(dependencies)
 
+## Image formats
+
+# This changes how find_xxx() commands work; the default is to find frameworks before
+# standard libraries or headers, but this can be a problem on systems that have Mono
+# installed, as it has a framework with the libjpeg and libpng  headers present -- so
+# CMake finds the headers from Mono but the libraries from Homebrew, and hilarity ensues.
+# Setting this to "last" means we always try the standard libraries before the frameworks.
+set(CMAKE_FIND_FRAMEWORK LAST)
+
+# TODO: these really belong in tools/, but CMake has a weird bug with $<TARGET_NAME_IF_EXISTS:...>
+# https://gitlab.kitware.com/cmake/cmake/-/issues/25033
+find_package(JPEG)
+find_package(PNG)
+
 ##
 # Declare options
 ##
@@ -114,7 +133,6 @@ cmake_dependent_option(
     "Halide_ENABLE_RTTI AND Halide_ENABLE_EXCEPTIONS" OFF
 )
 
-
 ##
 # Add source directories
 ##

dependencies/CMakeLists.txt

@@ -1,31 +1,9 @@
-##
-# Standard CMake find-module dependencies
-##
-
-set(THREADS_PREFER_PTHREAD_FLAG YES)
-find_package(Threads REQUIRED)
-set_target_properties(Threads::Threads PROPERTIES IMPORTED_GLOBAL TRUE)
-
-# TODO(https://github.com/halide/Halide/issues/5633): verify this is still correct / necessary for OpenGLCompute
-find_package(OpenGL)
-if (TARGET OpenGL::GL)
-    set_target_properties(OpenGL::GL PROPERTIES IMPORTED_GLOBAL TRUE)
-endif ()
-
-find_package(OpenMP)
-if (TARGET OpenMP::OpenMP_CXX)
-    set_target_properties(OpenMP::OpenMP_CXX PROPERTIES IMPORTED_GLOBAL TRUE)
-endif ()
-
 ##
 # Third-party dependencies in their own subdirectories
 ##
 
 add_subdirectory(llvm)
 
-add_subdirectory(jpeg)
-add_subdirectory(png)
-
 if (TARGET_SPIRV)
     add_subdirectory(spirv)
 endif()

tools/CMakeLists.txt

@@ -15,6 +15,9 @@ target_compile_options(regexp_replace PRIVATE $<$<CXX_COMPILER_ID:MSVC>:/wd4996>
 # Interface target for enabling PNG/JPEG support in Halide
 ##
 
+# TODO: if/when CMake fixes https://gitlab.kitware.com/cmake/cmake/-/issues/25033
+# then move find_package(PNG/JPEG) here.
+
 add_library(Halide_ImageIO INTERFACE)
 add_library(Halide::ImageIO ALIAS Halide_ImageIO)
 set_target_properties(Halide_ImageIO PROPERTIES EXPORT_NAME ImageIO)

tutorial/CMakeLists.txt

@@ -1,3 +1,5 @@
+find_package(OpenMP)
+
 configure_file(images/gray.png images/gray.png COPYONLY)
 configure_file(images/rgb.png images/rgb.png COPYONLY)