Rewrite Makefile shellcheck rule to find all shell scripts

Rewrite the find command which determines which files are
passed to shellcheck in order to include any file with a
shell-like shebang, since not all shell scripts have a
".sh" or ".ksh" extension

Signed-off-by: Gabriel A. Devenyi <gdevenyi@gmail.com>

Makefile.am

@@ -138,10 +138,9 @@ PHONY += shellcheck
 shellcheck:
 	@if type shellcheck > /dev/null 2>&1; then \
 		shellcheck --exclude=SC1090 --exclude=SC1117 --format=gcc \
-			$$(find ${top_srcdir}/scripts/*.sh -type f) \
-			$$(find ${top_srcdir}/cmd/zed/zed.d/*.sh -type f) \
-			$$(find ${top_srcdir}/cmd/zpool/zpool.d/* \
-			-type f ${filter_executable}); \
+			$$(find ${top_srcdir} -name '.git' -o -name 'build'  -prune -o \
+      -type f ! -name 'configure*' \
+      -exec bash -c 'awk "NR==1 && /sh/ {print FILENAME;}" "{}"' \;); \
 	else \
 		echo "skipping shellcheck because shellcheck is not installed"; \
 	fi