Detect jq at build time

Detect whether and where the jq utility is available at build time, so
that it can be used in system tests.  If the tool is not found, specific
checks employing it will be skipped.

bin/tests/system/isctest/vars/.build_vars/JQ.in

@@ -0,0 +1 @@
+@JQ@

bin/tests/system/isctest/vars/.build_vars/meson.build

@@ -28,6 +28,7 @@ configure_file(
 foreach name, prog : {
     'CURL': curl,
     'FSTRM_CAPTURE': fstrm_capture,
+    'JQ': jq,
     'NC': nc,
     'PERL': perl,
     'PYTEST': pytest,

bin/tests/system/isctest/vars/build.py

@@ -41,6 +41,7 @@ def load_vars_from_build_files() -> Dict[str, str]:
     for var in [
         "CURL",
         "FSTRM_CAPTURE",
+        "JQ",
         "NC",
         "PERL",
         "PYTEST",

meson.build

@@ -97,6 +97,7 @@ dtrace = find_program(['dtrace', dtrace_shim], required: false)
 curl = find_program('curl', required: false)
 fstrm_capture = find_program('fstrm_capture', required: false)
 git = find_program('git', required: false)
+jq = find_program('jq', required: false)
 nc = find_program('nc', required: false)
 python = find_program(['python3', 'python'], required: false)
 xsltproc = find_program('xsltproc', required: false)