cmake: Add library dependencies in Config.cmake.in when building stat…

…ic (AcademySoftwareFoundation#3552)

This fixes builds in projects that compile OpenImageIO as static
libraries and don't themselves depend on the image libraries used by
OpenImageIO such as PNG, JPEG or TIFF.

When OpenImageIO is built as a static library its targets have e.g.
PNG::PNG among INTERFACE_LINK_LIBRARIES. If the including project does
not itself depend on PNG, the following error would be observed:

Target "<...>" links to target "PNG::PNG" but the target was not found.
Perhaps a find_package() call is missing for an IMPORTED target, or an
ALIAS target is missing?

src/cmake/Config.cmake.in

@@ -17,6 +17,19 @@ if (NOT @OPENIMAGEIO_CONFIG_DO_NOT_FIND_IMATH@ AND NOT OPENIMAGEIO_CONFIG_DO_NOT
     endif ()
 endif ()
 
+if (NOT @BUILD_SHARED_LIBS@)
+    # This is required in static library builds, as e.g. PNG::PNG appears among
+    # INTERFACE_LINK_LIBRARIES. If the project does not know about PNG target, it will cause
+    # configuration error about unknown targets being linked in.
+    find_dependency(TIFF)
+    if (@JPEG_FOUND@)
+        find_dependency(JPEG)
+    endif()
+    if (@PNG_FOUND@)
+        find_dependency(PNG)
+    endif()
+endif ()
+
 # Compute the installation prefix relative to this file. Note that cmake files are installed
 # to ${CMAKE_INSTALL_LIBDIR}/cmake/${PROJECT_NAME} (see OIIO_CONFIG_INSTALL_DIR)
 get_filename_component(_CURR_INSTALL_LIBDIR "${CMAKE_CURRENT_LIST_DIR}/../../" ABSOLUTE)