Important changes in .inc files added by 'include <file_name>.inc' are ignored if .inc file is created after recipes are parsed. 

Changes in <filename>.inc will be taken into account if the .bb or .bbappend file it is in is changed in any way and thus requires a re-parse. 

Steps to reproduce:
1) get a clean build directory
2) add "include test_recipe.inc" to meta/recipes-devtools/m4/m4-native_1.4.17.bb
3) 'bitbake m4-native'
 - notice the stamp for m4-native do_install task in tmp/stamps . Track it along the way to see if the task id re-done.
4) create meta/recipes-devtools/m4/test_recipe.inc with the following contents:
do_install_append () {
        echo "Test if package rebuilds"
}

5) 'bitbake m4-native'
 - notice the changes are ignored; no tasks are re-done
 - notice m4-native do_install stamp is the same

6) add TEST_VAR = "1" in meta/recipes-devtools/m4/m4-native_1.4.17.bb
7) 'bitbake m4-native'
 - notice tasks after and including m4-native do_install are done
 - notice m4-native do_install stamp changed

8) Modify meta/recipes-devtools/m4/test_recipe.inc as follows:
do_install_append () {
        echo "The package should rebuild now"
}

9) 'bitbake m4-native'
 - notice tasks after and including m4-native do_install are done
 - notice m4-native do_install stamp changed

10) delete meta/recipes-devtools/m4/test_recipe.inc
11) 'bitbake -p'
12) recreate meta/recipes-devtools/m4/test_recipe.inc with the following contents:
do_install_append () {
        echo "Package should rebuild, but will not"
}

13) 'bitbake m4-native'
 - notice the changes are ignored; no tasks are re-done
 - notice m4-native do_install stamp is the same