diff --git a/Makefile b/Makefile
index c060363..2f6c0e9 100644
--- a/Makefile
+++ b/Makefile
@@ -22,6 +22,9 @@
 LC_NUMERIC=C
 export LC_COLLATE LC_NUMERIC
 
+# Avoid interference with shell env settings
+unexport GREP_OPTIONS
+
 # We are using a recursive build, so we need to do a little thinking
 # to get the ordering right.
 #