Re: make DejaGnu dependency optional

[Jose E. Marchesi]

> Trying to build GNU poke on CentOS 8.
>
> It does not have a package 'gc-devel', only 'gc' but that lacks <gc.h>. [1][2]
> OK, I can build and install 'gc' from source.
>
> It does not have a package 'json-c-devel', only 'json-c' but without the
> header files. [1][2] Oh well, it's optional.
>
> It does not have libtextstyle, only an older version of gettext [1]. Oh well,
> it's optional.
>
> It does not have a package 'dejagnu'. ('sudo dnf install dejagnu' gives
> "Error: Unable to find a match".) Which leads to a compilation
> failure:
>
> $ make check
> ...
> make[3]: Entering directory 
> '/home/bruno/poke-0.90/build-64/testsuite/poke.libpoke'
> gcc -DHAVE_CONFIG_H -I. -I../../../testsuite/poke.libpoke -I../../poke  
> -I../../gl -I../../../gl -I../../../common 
> -DTESTDIR=\"/home/bruno/poke-0.90/build-64/../testsuite/poke.libpoke\" 
> -DPKGDATADIR=\"/home/bruno/inst-x86_64-64/share/poke\" -I../../../libpoke 
> -I../../libpoke -I/home/bruno/inst-x86_64-64/include -Wall  -g -O2 -MT 
> values-values.o -MD -MP -MF .deps/values-values.Tpo -c -o values-values.o 
> `test -f 'values.c' || echo '../../../testsuite/poke.libpoke/'`values.c
> ../../../testsuite/poke.libpoke/values.c:26:10: fatal error: dejagnu.h: No 
> such file or directory
>  #include <dejagnu.h>
>           ^~~~~~~~~~~
> compilation terminated.
> make[3]: *** [Makefile:2063: values-values.o] Error 1
>
> Here, I'm too lazy to build it from source. Likewise on Solaris, which doesn't
> have it preinstalled either. This dependency should be optional. Here is a
> proposed patch. With it, the usual "./configure; make; make check" succeeds
> on CentOS 8 (although no tests were actually executed).
>
> The patch also fixes a confusion between a warning and an error: At the 'make'
> rule level, 'exit 77' does not skip a test; it makes the rule fail. So the
> message needs to say "ERROR", not "WARNING".

OK for master.
Thanks!