FindPackageModeMakefileTest: macOS needs isysroot flag

macOS/Xcode no longer populate /usr/include.
Building with the compiler directly instead of using /usr/bin/c++
can result in missing headers without the -isysroot flag.

Relates: #19885

Tests/FindPackageModeMakefileTest/CMakeLists.txt

@@ -19,6 +19,14 @@ if(UNIX  AND  "${CMAKE_GENERATOR}" MATCHES "Makefile" AND
     # configure a FindFoo.cmake so it knows where the library can be found
     configure_file(FindFoo.cmake.in ${CMAKE_CURRENT_BINARY_DIR}/FindFoo.cmake @ONLY)
 
+    # Need the -isysroot flag on recentish macOS after command line tools
+    # no longer provide headers in /usr/include
+    if(APPLE AND CMAKE_OSX_SYSROOT)
+      set(__EXTRA_OSX_SYSROOT_FLAGS "-isysroot ${CMAKE_OSX_SYSROOT}")
+    else()
+      set(__EXTRA_OSX_SYSROOT_FLAGS "")
+    endif()
+
     # now set up the test:
     file(GENERATE OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/cmakeExecutable.mk"
       CONTENT "CMAKE = \"$<TARGET_FILE:cmake>\"\n"

Tests/FindPackageModeMakefileTest/Makefile.in

@@ -5,6 +5,7 @@ CMAKE_CURRENT_BINARY_DIR = "@CMAKE_CURRENT_BINARY_DIR@"
 CMAKE_CXX_COMPILER = "@CMAKE_CXX_COMPILER@"
 CMAKE_CXX_COMPILER_ID = "@CMAKE_CXX_COMPILER_ID@"
 CMAKE_CXX_FLAGS = @CMAKE_CXX_FLAGS@
+__EXTRA_OSX_SYSROOT_FLAGS = @__EXTRA_OSX_SYSROOT_FLAGS@
 
 CMAKE_FOO = $(CMAKE) --find-package -DCMAKE_MODULE_PATH=$(CMAKE_CURRENT_BINARY_DIR) -DNAME=Foo -DLANGUAGE=CXX -DCOMPILER_ID=$(CMAKE_CXX_COMPILER_ID)
 
@@ -15,7 +16,7 @@ all: pngtest
 main.o: clean main.cpp
 	@$(CMAKE_FOO) -DMODE=COMPILE >$(tmp)
 	@foo="`cat $(tmp)`"; \
-	 printf '"%s" %s %s -c main.cpp\n' $(CMAKE_CXX_COMPILER) "$(CMAKE_CXX_FLAGS)" "$$foo" >$(tmp)
+	 printf '"%s" %s %s %s -c main.cpp\n' $(CMAKE_CXX_COMPILER) "$(CMAKE_CXX_FLAGS)" "$(__EXTRA_OSX_SYSROOT_FLAGS)" "$$foo" >$(tmp)
 	@cat $(tmp)
 	@sh $(tmp)
 	@rm -f $(tmp)