Add check for HB_EXTERNs

https://github.com/behdad/harfbuzz/pull/555

src/Makefile.am

@@ -300,6 +300,7 @@ check: harfbuzz.def # For check-defs.sh
 dist_check_SCRIPTS = \
 	check-c-linkage-decls.sh \
 	check-defs.sh \
+	check-externs.sh \
 	check-header-guards.sh \
 	check-includes.sh \
 	check-libstdc++.sh \

src/check-externs.sh

@@ -0,0 +1,21 @@
+#!/bin/sh
+
+LC_ALL=C
+export LC_ALL
+
+test -z "$srcdir" && srcdir=.
+stat=0
+
+test "x$HBHEADERS" = x && HBHEADERS=`cd "$srcdir"; find . -maxdepth 1 -name 'hb*.h'`
+test "x$EGREP" = x && EGREP='grep -E'
+
+
+echo 'Checking that all public symbols are exported with HB_EXTERN'
+
+for x in $HBHEADERS; do
+	test -f "$srcdir/$x" && x="$srcdir/$x"
+	$EGREP -B1 '^hb_' "$x" | $EGREP -E -v '^(--|hb_|HB_EXTERN )' -A1
+done |
+grep . >&2 && stat=1
+
+exit $stat