tests: don't try to build llvm test if dep is unfound

dragonfly BSD doesn't ship dynamic modules for LLVM so static linking is
required.

test cases/frameworks/15 llvm/meson.build

@@ -20,9 +20,10 @@ foreach static : [true, false]
     'llvm',
     modules : ['bitwriter', 'asmprinter', 'executionengine', 'target',
                'mcjit', 'nativecodegen'],
-    required : true,
+    required : false,
     static : static,
   )
+  if llvm_dep.found()
     name = static ? 'static' : 'dynamic'
     executable(
       'sum-@0@'.format(name),
@@ -33,4 +34,5 @@ foreach static : [true, false]
         meson.get_compiler('c').find_library('dl', required : false),
       ]
     )
+  endif
 endforeach