travis: check for doxygen warnings

This introduces a script that will check for doxygen warnings in newly
committed or edited files.

dist/tools/doccheck/check.sh

+#!/bin/sh
+
+TMP1=$(mktemp)
+TMP2=$(mktemp)
+
+# If no branch but an option is given, unset BRANCH.
+# Otherwise, consume this parameter.
+BRANCH="${1}"
+if echo "${BRANCH}" | grep -q '^-'; then
+    BRANCH=""
+else
+    if [ -n "${BRANCH}" ]; then
+        shift 1
+    fi
+fi
+
+# If the --diff-filter option is given, consume this parameter.
+# Set the default DIFFFILTER option otherwise.
+DIFFFILTER="${1}"
+if echo "${DIFFFILTER}" | grep -q '^--diff-filter='; then
+    shift 1
+else
+    DIFFFILTER="--diff-filter=ACMR"
+fi
+
+# select files to check
+if [ -z "${BRANCH}" ]; then
+    git ls-tree -r --full-tree --name-only HEAD | grep -E '\.([sSch]|cpp)$' | sort | uniq > ${TMP1}
+else
+    git diff ${DIFFFILTER} --name-only ${BRANCH} | grep -E '\.([sSchp]|cpp)$' | sort | uniq > ${TMP1}
+fi
+
+make doc 2>&1 | grep warning | grep -E '^\/' | sed "s#${PWD}\/\([^:]*\).*#\1#" | sort | uniq > ${TMP2}
+
+WARNINGS=$(comm -1 -2 ${TMP1} ${TMP2})
+
+rm ${TMP_ERR} ${TMP_WARN} ${TMP_DOC}
+    
+if [ -n "${WARNINGS}" ]
+then
+echo "WARNING: The following modified files generate doxygen warnings:"
+echo "${WARNINGS}"
+fi
+
+if [ -n "${WARNINGS}" ]
+then
+    echo "The following files generate doxygen warnings: ${WARNINGS}"
+    exit 2
+else
+    exit 0
+fi