add a simple code coverage generation script

The lcov tools (http://ltp.sourceforge.net/coverage/lcov.php) are
required to generate reports.

Original idea from this PR, thanks to Jim Barlow for doing the initial
integration work.

https://github.com/libressl-portable/portable/pull/58

gen-coverage-report.sh

@@ -0,0 +1,37 @@
+#!/bin/sh
+
+VERSION=$(cat VERSION)
+DESTDIR=libressl-coverage-$VERSION
+
+echo "This will generate a code coverage report under $DESTDIR"
+echo
+
+if [ "x$(which lcov)" = "x" ]; then
+	echo "'lcov' is required but not found!"
+	exit 1
+fi
+
+if [ "x$(which genhtml)" = "x" ]; then
+	echo "'genhtml' is required but not found!"
+	exit 1
+fi
+
+find -name '*.gcda' -o -name '*.gcno' -delete
+rm -fr $DESTDIR
+
+echo "Configuring to build with code coverage support"
+./configure --enable-libtls CFLAGS='-O0 -fprofile-arcs -ftest-coverage'
+
+echo "Running all code paths"
+make clean
+make check
+
+echo "Generating report"
+mkdir -p $DESTDIR
+find tests -name '*.gcda' -o -name '*.gcno' -delete
+lcov --directory . --capture --output-file $DESTDIR/coverage.tmp \
+    --test-name "LibreSSL $VERSION"
+genhtml --prefix . --output-directory $DESTDIR \
+    --title "LibreSSL $VERSION" --legend --show-detail $DESTDIR/coverage.tmp
+
+echo "Code coverage report is available under $DESTDIR"