diff --git a/src/Makevars.ucrt b/src/Makevars.ucrt
index 1a2e101..b511345 100644
--- a/src/Makevars.ucrt
+++ b/src/Makevars.ucrt
@@ -1,2 +1,8 @@
-CRT=-ucrt
-include Makevars.win
+# Used on rtools43 and up
+PKG_LIBS = $(shell pkg-config --libs gmpxx mpfr)
+
+# Fallback for older rtools without pkgconfig
+ifeq (,$(PKG_LIBS))
+  CRT=-ucrt
+  include Makevars.win
+endif