Make the xml pipespec file VPATH safe, should fix build failing in CI

Author: snomos

tools/grammarcheckers/pipespec.xml.in

@@ -72,15 +72,15 @@
             language="@GLANG2@"
             type="Grammar error, dev version">
     <tokenize><tokenizer n="tokeniser-gramcheck-gt-desc.pmhfst"/></tokenize>
-    <cg><grammar n="mwe-dis.cg3"/></cg>
+    <cg><grammar n="@srcdir@/../tokenisers/mwe-dis.cg3"/></cg>
     <mwesplit/>
     <cgspell>
       <lexicon n="acceptor.default.hfst"/>
       <errmodel n="errmodel.default.hfst"/>
     </cgspell>
-    <cg><grammar n="disambiguator.cg3"/></cg>
-    <cg><grammar n="spellchecker.cg3"/></cg>
-    <cg><grammar n="grammarchecker.cg3"/></cg>
+    <cg><grammar n="@srcdir@/../../src/cg3/disambiguator.cg3"/></cg>
+    <cg><grammar n="@srcdir@/spellchecker.cg3"/></cg>
+    <cg><grammar n="@srcdir@/grammarchecker.cg3"/></cg>
     <suggest>
       <generator n="generator-gramcheck-gt-norm.hfstol"/>
       <messages n="errors.xml"/>