set an exit code in make_doc (#5835)

For that,
- redirect the `Info` output generated by GAPDoc functions to a string,
- extract the messages that describe warnings; according to
  frankluebeck/GAPDoc/commit/826c091491438c1f6ba1f330d17914f45b4bd3af,
  these are the lines starting with `"#W "`,
- omit the LaTeX warnings about overfull boxes (I think we do not want
  to force ourselves to create manuals without these warning),
- report failure if the set of remaining warnings is nonempty.

doc/make_doc.in

@@ -4,7 +4,7 @@ set -e
 set -o pipefail
 
 GAP=@abs_top_builddir@/gap
-GAPARGS="-b -m 1g -x 80 -q --quitonbreak"
+GAPARGS="-b -m 1g -x 80 -q --quitonbreak -r"
 if [ "$1" == "nopdf" ]; then
   NOPDF=", \"nopdf\""
 else
@@ -20,6 +20,13 @@ books:=["ref", "tut", "hpc", "dev"];;
 latexOpts := rec(Maintitlesize := "\\\\fontsize{36}{38}\\\\selectfont");;
 UpdateXMLForUserPreferences();
 for run in [1,2] do
+  # collect output messages separately in both runs
+  outputstring:= "";;
+  outputstream:= OutputTextString(outputstring, true);;
+  SetPrintFormattingStatus(outputstream, false);
+  SetInfoOutput(InfoGAPDoc, outputstream);
+  SetInfoOutput(InfoWarning, outputstream);
+
   for book in books do
     path := Concatenation(base, "/doc/", book);
     dir := Directory(path);
@@ -29,10 +36,10 @@ for run in [1,2] do
         continue;
     fi;
 
-    Print("----------------------------\n");
-    Print("Building GAP manual '",book,"' at ",path,"\n");
-    Print("Run ",run," of 2\n");
-    Print("----------------------------\n");
+    AppendTo(outputstream, "----------------------------\n");
+    AppendTo(outputstream, "Building GAP manual '",book,"' at ",path,"\n");
+    AppendTo(outputstream, "Run ",run," of 2\n");
+    AppendTo(outputstream, "----------------------------\n");
 
     # for the reference manual, extra list of source files to scan
     if book = "ref" then
@@ -65,5 +72,21 @@ for run in [1,2] do
     SetGapDocLaTeXOptions("color", latexOpts);
     MakeGAPDocDoc( path, "main.xml", files, book, "../..", "MathJax" $NOPDF);;
   od;
+
+  CloseStream(outputstream);
+  UnbindInfoOutput(InfoGAPDoc);
+  UnbindInfoOutput(InfoWarning);
+  Print(outputstring);
 od;
+
+# evaluate the outputs for the second run
+outputstring:= ReplacedString(outputstring, "\c", "");;
+errors:= Filtered(SplitString(outputstring, "\n"),
+           x -> StartsWith(x, "#W ") and x <> "#W There are overfull boxes:");;
+if Length(errors) = 0 then
+  QuitGap(true);
+else
+  Print(errors, "\n");
+  QuitGap(false);
+fi;
 EOF