diff --git a/src/m4/bitcoin_subdir_to_include.m4 b/src/m4/bitcoin_subdir_to_include.m4
index 9b37a75ef..66f106c7d 100644
--- a/src/m4/bitcoin_subdir_to_include.m4
+++ b/src/m4/bitcoin_subdir_to_include.m4
@@ -5,8 +5,10 @@ AC_DEFUN([BITCOIN_SUBDIR_TO_INCLUDE],[
     AC_MSG_RESULT([default])
   else
     echo "#include <$2$3.h>" >conftest.cpp
-    newinclpath=`${CXXCPP} -M conftest.cpp 2>/dev/null | [ tr -d '\\n\\r\\\\' | sed -e 's/^.*[[:space:]:]\(\/[^[:space:]]*\)]$3[\.h[[:space:]].*$/\1/' -e t -e d`]
+    newinclpath=`${CXXCPP} ${CPPFLAGS} -M conftest.cpp 2>/dev/null | [ tr -d '\\n\\r\\\\' | sed -e 's/^.*[[:space:]:]\(\/[^[:space:]]*\)]$3[\.h[[:space:]].*$/\1/' -e t -e d`]
     AC_MSG_RESULT([${newinclpath}])
-    eval "$1=\"\$$1\"' -I${newinclpath}'"
+    if test "x${newinclpath}" != "x"; then
+      eval "$1=\"\$$1\"' -I${newinclpath}'"
+    fi
   fi
 ])