autotools: fix subdir m4 check, never let it return -I

If BDB_CPPFLAGS returns only "-I", the next argument sent to the preprocessor
is treated as a path. There are 2 fixes here:

1. Check in CPPFLAGS, as a user might have manually passed a path to check.
2. Ensure the value is not empty before setting BDB_CPPFLAGS to "-I value"

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([${newinclpath}])
+    if test "x${newinclpath}" != "x"; then
       eval "$1=\"\$$1\"' -I${newinclpath}'"
     fi
+  fi
 ])