Force use of -O0 -g in 2019/endoh

Previously only -g was after ${CFLAGS} but it needs to have -O0 in case
someone decides to run something like

    make clobber OPT= all

Use of OPT+= in the Makefile will not fix that problem. Thus if someone
does NOT mess with OPT or any other variables that are added to CFLAGS
it will look like:

     -O0 -g -O0 -g prog.c -o prog

Previously it was just '-g' and no '-O0' but if someone were to do
instead

    make clobber OPT=-O3 all

it would have messed that up.

2019/endoh/README.md

@@ -4,6 +4,9 @@
 make
 ```
 
+NOTE: the use of the C compiler optimiser was disabled and `-g` was enabled for
+this entry because it is SUPPOSED to crash and it needs debugging symbols.
+
 
 ### Bugs and (Mis)features: