building without json prerequisite

[Bruno Haible]

Hi,

The HACKING file says that the JSON-C library is optional:
  "If this library is not found, poke will build without MI support."

That's not what happens now. Instead, a build failure occurs during
"make check":

gcc -DHAVE_CONFIG_H -I. -I../../poke  -I../../gl -I../../gl -I../../poke 
-I../../poke -I/inst-x86_64-64/include -Wall  -g -O2 -MT 
../../poke/mi_json-pk-mi-json.o -MD -MP -MF 
../../poke/.deps/mi_json-pk-mi-json.Tpo -c -o ../../poke/mi_json-pk-mi-json.o 
`test -f '../../poke/pk-mi-json.c' || echo './'`../../poke/pk-mi-json.c
../../poke/pk-mi-json.c:23:18: fatal error: json.h: No such file or directory
compilation terminated.
make[3]: *** [Makefile:1858: ../../poke/mi_json-pk-mi-json.o] Error 1
make[3]: Leaving directory '/POKE/poke/testsuite/poke.mi-json'
make[2]: *** [Makefile:1963: check-am] Error 2

The attached patch
1) avoids building code that won't build,
2) avoids a "make check" error message
  Running /POKE/poke/testsuite/poke.mi-json/mi-json.exp ...
  ERROR: The executable, "poke.mi-json/mi-json" is missing
  ERROR: mi-json had an execution error

With this, the continuous integration should work fine again.

OK to push?

0001-Fix-build-failure-during-make-check-without-libjson-.patch
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