diff --git a/cadpli/configure.in b/cadpli/configure.in
index 31d7e1563..ca923ce6c 100644
--- a/cadpli/configure.in
+++ b/cadpli/configure.in
@@ -34,8 +34,8 @@ AC_SUBST(DLLIB)
 # Darwin requires -no-cpp-precomp
 case "${host}" in
     *-*-darwin*)
-	CPPFLAGS="-no-cpp-precomp"
-	CFLAGS="-no-cpp-precomp"
+	CPPFLAGS="-no-cpp-precomp $CPPFLAGS"
+	CFLAGS="-no-cpp-precomp $CFLAGS"
 	;;
 esac
 
diff --git a/configure.in b/configure.in
index 9610700ec..7ca8ac85d 100644
--- a/configure.in
+++ b/configure.in
@@ -135,8 +135,8 @@ AC_MSG_RESULT($EXTRALIBS)
 # Darwin requires -no-cpp-precomp
 case "${host}" in
     *-*-darwin*)
-	CPPFLAGS="-no-cpp-precomp"
-	CFLAGS="-no-cpp-precomp"
+	CPPFLAGS="-no-cpp-precomp $CPPFLAGS"
+	CFLAGS="-no-cpp-precomp $CFLAGS"
 	;;
 esac
 
diff --git a/libveriuser/configure.in b/libveriuser/configure.in
index 99a9ce02e..49bdaf3d5 100644
--- a/libveriuser/configure.in
+++ b/libveriuser/configure.in
@@ -10,8 +10,8 @@ AC_EXEEXT
 # Darwin requires -no-cpp-precomp
 case "${host}" in
     *-*-darwin*)
-	CPPFLAGS="-no-cpp-precomp"
-	CFLAGS="-no-cpp-precomp"
+	CPPFLAGS="-no-cpp-precomp $CPPFLAGS"
+	CFLAGS="-no-cpp-precomp $CFLAGS"
 	;;
 esac
 
diff --git a/tgt-fpga/configure.in b/tgt-fpga/configure.in
index ea9e084b3..6ce1621a4 100644
--- a/tgt-fpga/configure.in
+++ b/tgt-fpga/configure.in
@@ -16,8 +16,8 @@ AC_CHECK_HEADERS(malloc.h)
 # Darwin requires -no-cpp-precomp
 case "${host}" in
     *-*-darwin*)
-	CPPFLAGS="-no-cpp-precomp"
-	CFLAGS="-no-cpp-precomp"
+	CPPFLAGS="-no-cpp-precomp $CPPFLAGS"
+	CFLAGS="-no-cpp-precomp $CFLAGS"
 	;;
 esac
 
diff --git a/tgt-vvp/configure.in b/tgt-vvp/configure.in
index b3c9cc5b7..32ca851cf 100644
--- a/tgt-vvp/configure.in
+++ b/tgt-vvp/configure.in
@@ -16,8 +16,8 @@ AC_CHECK_HEADERS(malloc.h)
 # Darwin requires -no-cpp-precomp
 case "${host}" in
     *-*-darwin*)
-	CPPFLAGS="-no-cpp-precomp"
-	CFLAGS="-no-cpp-precomp"
+	CPPFLAGS="-no-cpp-precomp $CPPFLAGS"
+	CFLAGS="-no-cpp-precomp $CFLAGS"
 	;;
 esac
 
diff --git a/vpip/configure.in b/vpip/configure.in
index e48624e08..abc307fe5 100644
--- a/vpip/configure.in
+++ b/vpip/configure.in
@@ -12,8 +12,8 @@ AC_EXEEXT
 # Darwin requires -no-cpp-precomp
 case "${host}" in
     *-*-darwin*)
-	CPPFLAGS="-no-cpp-precomp"
-	CFLAGS="-no-cpp-precomp"
+	CPPFLAGS="-no-cpp-precomp $CPPFLAGS"
+	CFLAGS="-no-cpp-precomp $CFLAGS"
 	;;
 esac
 
diff --git a/vvp/configure.in b/vvp/configure.in
index 88bad9e77..abf2a20fc 100644
--- a/vvp/configure.in
+++ b/vvp/configure.in
@@ -58,8 +58,8 @@ AC_SUBST(DLLIB)
 # Darwin requires -no-cpp-precomp
 case "${host}" in
     *-*-darwin*)
-	CPPFLAGS="-no-cpp-precomp"
-	CFLAGS="-no-cpp-precomp"
+	CPPFLAGS="-no-cpp-precomp $CPPFLAGS"
+	CFLAGS="-no-cpp-precomp $CFLAGS"
 	;;
 esac