org: detect C++ errors

* doc/org/g++wrap.in: Save error messages.
* doc/Makefile.am: Display them at the end of the compilation.

doc/org/g++wrap.in

@@ -1,6 +1,17 @@
 #!/bin/sh
 # This is a wrapper around the compiler, to ensure that the code
 # example run from the org-mode file are all linked with Spot.
-exec @top_builddir@/libtool link @CXX@ -std=c++11 -Wall \
-     -I@abs_top_builddir@ -I@abs_top_srcdir@ -I@abs_top_srcdir@/buddy/src \
-     "$@" @abs_top_builddir@/spot/libspot.la
+#
+# Also we save errors to org.errors, so that we can detect issues
+# after org-mode has exported everything.  Otherwise these errors
+# would go undetected.
+@top_builddir@/libtool link @CXX@ @CXXFLAGS@ -Wall -Werror \
+ -I@abs_top_builddir@ -I@abs_top_srcdir@ -I@abs_top_srcdir@/buddy/src \
+ "$@" @abs_top_builddir@/spot/libspot.la 2> errors.$$
+code=$?
+if test $code -ne 0 && test -s errors.$$; then
+    cat errors.$$ >>org.errors
+    cat errors.$$ >&2
+fi
+rm -f errors.$$
+exit $code