Make started complaining about UTF-8 files

Beginning 2016 I set up a basic archlinux system for running an openframeworks app. I documented the process and since then the app is running (it’s part of a temporary set up where nothing gets changed); it just worked out fine.
A while ago (mid April) I tried to set up a basic archlinux system for another openframeworks app again; for setting up I used the same PC and I followed the steps in my documentation (basically the “Beginners’ guide” of archlinux plus a few additionals), but compiling the openframeworks code on that fresh archlinux system fails (also the code that was working beginning 2016; but the example apps are compiling, which files are ‘pure’ ASCII encoded).
The error message points me to:

In file included from /usr/include/c++/5.3.0/iomanip:43:0, from /home/of/of_v0.9.3_linux64_release/libs/openFrameworks/utils/ofConstants.h:388, from /home/of/of_v0.9.3_linux64_release/libs/openFrameworks/ofMain.h:5, from /home/of/of_v0.9.3_linux64_release/apps/myApps/poesiekubus/src/main.cpp:1: /./bin/locale:1:1: Fehler: verirrtes »\177« im Programm ELF > Ð @ @ ø @ 8 @ @ @ @ @ @ ø ø 8 8@ 8@ @ @ üc üc n n` n` ä ! n n` n` à à X X@ X@ D D Påtd |[ |[@ |[@ D D Qåtd Råtd n n` n` /usr/lib/ld-linux-x86-64.so.2 GNU GNU “N(€€ž&¤'¹\ï‘í Ðâµ  K < B 4 . J : C ' 8 I E G ; a 5 2 * $ @ & ( D A H # ) - 3 ! e / , 9 + 7 1 = ? 6 F % 0 ^
[… a lot of strange symbols follow with other errors about “verirrte”/strayed characters in the program]

iomanip at line 43 points at "#include " and in there it goes further to conversion … but there I lose trace.

the output of locale is:

LANG=de_AT.UTF-8 LC_CTYPE="de_AT.UTF-8" LC_NUMERIC="de_AT.UTF-8" LC_TIME="de_AT.UTF-8" LC_COLLATE="de_AT.UTF-8" LC_MONETARY="de_AT.UTF-8" LC_MESSAGES="de_AT.UTF-8" LC_PAPER="de_AT.UTF-8" LC_NAME="de_AT.UTF-8" LC_ADDRESS="de_AT.UTF-8" LC_TELEPHONE="de_AT.UTF-8" LC_MEASUREMENT="de_AT.UTF-8" LC_IDENTIFICATION="de_AT.UTF-8" LC_ALL=

and the files are for example:

It seems that character encoding (UTF-8 - ASCII?) does not work anymore and I could partially fix it by recoding the names of image files and then compiling gets a bit further, but it does not complete and it is not practical.
Also upgrading to gcc-6.1 didn’t solve that issue and since now I didn’t manage to check out, which package/configuration has changed, causing the build process to break.
Is there a way how to check what has changed and how to fix that issue?
All the best

Tracing the issues of “locale” was not successful … but a quite straight
solution is character encoding the src files from “utf8” to “ascii”:

iconv -f utf8 -t ascii//TRANSLIT ofApp.cpp_utf8 > ofApp.cpp_ascii

Then building works out and it solved that issue.