make test dependencies optional

test/meson.build

 # add checkpatch syntax-check target
 checkpatch = find_program('checkpatch.pl', 'checkpatch', required : false)
+if checkpatch.found()
   checkpatch_args = [
     '--no-tree', '-f',
     '--show-types',
@@ -8,9 +9,11 @@ checkpatch_args = [
   ]
   run_target('syntax-check',
    command : [checkpatch, checkpatch_args, main, cmplib_sources])
+endif
 
 # add cppcheck inspector target
 cppcheck = find_program('cppcheck', required : false)
+if cppcheck.found()
   cppcheck_args = [
     main, cmplib_sources,
     '--clang',
@@ -25,9 +28,12 @@ cppcheck_args = [
   run_target('inspector',
     command : [cppcheck, cppcheck_args]
   )
-
-cunit_dep = dependency('cunit')
+endif
 
 subdir('cmp_tool')
+
+cunit_dep = dependency('cunit')
+if cunit_dep.found()
   subdir('cmp_icu')
+endif