Do not force the install libdir (e.g. /opt/local/lib) onto the g-ir-scanner's
DYLD_LIBRARY_PATH on macOS. DYLD_LIBRARY_PATH is a forced override, so having
the MacPorts libdir on it makes Apple system frameworks (ImageIO) bind their
JPEG symbols against MacPorts libJPEG.dylib, which lacks them, e.g.
  dyld: Symbol not found: __cg_jpeg_resync_to_restart
    Referenced from: .../ImageIO.framework/.../ImageIOOriginal
    Expected in: /opt/local/lib/libJPEG.dylib
The just-built libraries still need the build dirs on the path; the installed
libraries are located via their install names / DYLD_FALLBACK_LIBRARY_PATH.

--- cmake/modules/GObjectIntrospection.cmake.orig
+++ cmake/modules/GObjectIntrospection.cmake
@@ -266,14 +266,34 @@
 		_loader_library_path_cmake
 	)
 
-	set(_extra_loader_library_path_cmake
-		${CMAKE_BINARY_DIR}
-		${CMAKE_BINARY_DIR}/src
-		${CMAKE_CURRENT_BINARY_DIR}
-		${${gir_extra_libdirs_var}}
-		${LIB_INSTALL_DIR}
-		${_loader_library_path_cmake}
-	)
+	# On macOS the scanner env sets DYLD_LIBRARY_PATH (a forced override, not a
+	# fallback). Putting the install libdir (e.g. /opt/local/lib) on it makes
+	# dyld satisfy Apple framework symbols from MacPorts libraries, which breaks
+	# with errors such as
+	#   dyld: Symbol not found: __cg_jpeg_resync_to_restart
+	#     Expected in: /opt/local/lib/libJPEG.dylib
+	# when the scanner subprocess transitively loads ImageIO. The just-built
+	# libraries still need the build dirs here; the installed ones are found via
+	# their install names / DYLD_FALLBACK_LIBRARY_PATH, so drop the install
+	# libdir from the forced path on Apple.
+	if(APPLE)
+		set(_extra_loader_library_path_cmake
+			${CMAKE_BINARY_DIR}
+			${CMAKE_BINARY_DIR}/src
+			${CMAKE_CURRENT_BINARY_DIR}
+			${${gir_extra_libdirs_var}}
+			${_loader_library_path_cmake}
+		)
+	else(APPLE)
+		set(_extra_loader_library_path_cmake
+			${CMAKE_BINARY_DIR}
+			${CMAKE_BINARY_DIR}/src
+			${CMAKE_CURRENT_BINARY_DIR}
+			${${gir_extra_libdirs_var}}
+			${LIB_INSTALL_DIR}
+			${_loader_library_path_cmake}
+		)
+	endif(APPLE)
 
 	file(TO_NATIVE_PATH
 		"${_extra_loader_library_path_cmake}"