Thx to matthiaskrgr for working with me on this issue. :)

What was tried...
git clean -dfx; git reset --hard HEAD; ./autogen.sh; ./configure; make
result: no issues - git diff shows no changes in files

git clean -dfx; git reset --hard HEAD; ./autogen.sh; ./configure; make -j6
(have bulldozer 6-core cpu)
result: make fails complaining about erros in building gmo files
errors...
----------
mv: cannot stat ‘t-de.gmo’: No such file or directory
Makefile:209: recipe for target 'de.gmo' failed
make[4]: *** [de.gmo] Error 1
make[4]: Leaving directory
'/home/me/localbuild/SRCs/freedroid/freedroid-code/po/po-dialogs'
Makefile:246: recipe for target 'stamp-po' failed
make[3]: *** [stamp-po] Error 2
make[3]: *** Waiting for unfinished jobs....
make[4]: Entering directory
'/home/me/localbuild/SRCs/freedroid/freedroid-code/po/po-dialogs'
installing fr.gmo as ../../locale/fr/LC_MESSAGES/freedroidrpg-dialogs.mo
installing de.gmo as ../../locale/de/LC_MESSAGES/freedroidrpg-dialogs.mo
make[4]: Leaving directory
'/home/me/localbuild/SRCs/freedroid/freedroid-code/po/po-dialogs'
touch stamp-li
make[3]: Leaving directory
'/home/me/localbuild/SRCs/freedroid/freedroid-code/po/po-dialogs'
Makefile:385: recipe for target 'all-recursive' failed
make[2]: *** [all-recursive] Error 1
make[2]: Leaving directory '/home/me/localbuild/SRCs/freedroid/freedroid-code/po'
Makefile:536: recipe for target 'all-recursive' failed
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory '/home/me/localbuild/SRCs/freedroid/freedroid-code'
Makefile:432: recipe for target 'all' failed
make: *** [all] Error 2
----------
run make clean; make; git status
---> file changes in graphics/floor_tiles/ and graphics/obstacle files are found

git clean -dfx; git reset --hard HEAD; ./autogen.sh; ./configure; make -j4
result: no issues - git diff shows no changes in files

this error is "repeatable"