make DejaGnu dependency optional

[Bruno Haible]

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".


[1] http://mirror.centos.org/centos/8/BaseOS/x86_64/os/Packages/
[2] http://mirror.centos.org/centos/8/AppStream/x86_64/os/Packages/

0001-Make-DejaGnu-dependency-optional.patch
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