tools/check-exports.sh: use ${NM} when available

This way you can export NM to use a prefixed tool

https://bugzilla.gnome.org/show_bug.cgi?id=755540

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>

tools/check-exports.sh

@@ -12,7 +12,7 @@ TMPFILE="$(mktemp .nm-check-exports.XXXXXX)"
 
 
 get_syms() {
-    nm "$1" |
+    ${NM:-nm} "$1" |
     sed -n 's/^[[:xdigit:]]\+ [DT] //p' |
     sort
 }