Make buildtests.in more robust

scripts/buildtests.in

diff --git a/scripts/buildtests.in b/scripts/buildtests.in
index 526d5b7..ab9c18f 100755
--- a/scripts/buildtests.in
+++ b/scripts/buildtests.in
@@ -10,7 +10,7 @@
 fi
 
 TESTSLIST="@EIGEN_TESTS_LIST@"
-targets_to_make=`echo "$TESTSLIST" | egrep "$1" | xargs echo`
+targets_to_make=$(echo "$TESTSLIST" | grep -E "$1" | xargs echo)
 
 if [ -n "${EIGEN_MAKE_ARGS:+x}" ]
 then