Add SDK usr/lib to LIBRARY_PATH on Mac for Tcl

Initial clone is failing on an in-build-dir checkout, unable to compile
because it can't find the System lib.

tcl/CMakeLists.txt

@@ -49,6 +49,9 @@ if (ENABLE_TCL)
   # succeed
   if (APPLE)
     set(RPATH_CMD "COMMAND;install_name_tool;-add_rpath;@executable_path/../${LIB_DIR};${CMAKE_BUNDLE_INSTALL_PREFIX}/${BIN_DIR}/tclsh${TCL_VERSION}${CMAKE_EXECUTABLE_SUFFIX}")
+    # Apparently LIBRARY_PATH is sometimes needed on Mac - see
+    # https://stackoverflow.com/a/70676858
+    execute_process(COMMAND xcrun --sdk macosx --show-sdk-path OUTPUT_VARIABLE SDKROOT OUTPUT_STRIP_TRAILING_WHITESPACE)
   endif (APPLE)
 
   configure_file(${CMAKE_CURRENT_SOURCE_DIR}/tcl_configure.cmake.in ${CMAKE_CURRENT_BINARY_DIR}/tcl_configure.cmake @ONLY)

tcl/tcl_configure.cmake.in

@@ -1,4 +1,8 @@
 if (NOT WIN32)
+  if(APPLE)
+    # https://stackoverflow.com/a/70676858
+    set(ENV{LIBRARY_PATH} "@SDKROOT@/usr/lib")
+  endif(APPLE)
   set(ENV{LD_LIBRARY_PATH} "@CMAKE_BUNDLE_INSTALL_PREFIX@/@LIB_DIR@")
   set(ENV{CPPFLAGS} "-I@CMAKE_BUNDLE_INSTALL_PREFIX@/@INCLUDE_DIR@")
   set(ENV{LDFLAGS} "-L@CMAKE_BUNDLE_INSTALL_PREFIX@/@LIB_DIR@")