--- ./configure.ac.orig	2021-02-16 13:00:47 +0000
+++ ./configure.ac	2021-02-16 13:01:44 +0000
@@ -116,12 +116,9 @@
 AC_SUBST(BOOST_CPPFLAGS)
 
 dnl How to specify include directories that may be system directories.
-# -I should not be used on system directories (GCC)
-if test "$GCC" = yes; then
-    ISYSTEM="-isystem "
-else
+
     ISYSTEM="-I"
-fi
+
 AC_MSG_NOTICE(ISYSTEM: $ISYSTEM)
 
 dnl Info for building shared libraries ... in order to run the examples