Detect xmllint at build time

Detect whether and where the xmllint 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/XMLLINT.in

@@ -0,0 +1 @@
+@XMLLINT@

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

@@ -34,6 +34,7 @@ foreach name, prog : {
     'PYTEST': pytest,
     'PYTHON': python,
     'SHELL': sh,
+    'XMLLINT': xmllint,
     'XSLTPROC': xsltproc,
 }
     if prog.found()

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

@@ -48,6 +48,7 @@ def load_vars_from_build_files() -> Dict[str, str]:
         "PYTHON",
         "SHELL",
         "TOP_SRCDIR",
+        "XMLLINT",
         "XSLTPROC",
     ]:
         var_file = var_dir / var

meson.build

@@ -100,6 +100,7 @@ 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)
+xmllint = find_program('xmllint', required: false)
 xsltproc = find_program('xsltproc', required: false)
 pytest = find_program(
     ['pytest-3', 'py.test-3', 'pytest', 'py.test', 'pytest-pypy'],