diff --git a/icepack/icepack.cc b/icepack/icepack.cc
index 4eb0305..b5db4c5 100644
--- a/icepack/icepack.cc
+++ b/icepack/icepack.cc
@@ -16,7 +16,10 @@
 //  OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 //
 
+#if !defined(_WIN32) && !defined(_GNU_SOURCE)
+// for vasprintf()
 #define _GNU_SOURCE
+#endif
 
 #include <set>
 #include <tuple>
diff --git a/icetime/icetime.cc b/icetime/icetime.cc
index 1d0549c..affc56d 100644
--- a/icetime/icetime.cc
+++ b/icetime/icetime.cc
@@ -14,7 +14,10 @@
 //  OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 //
 
+#if !defined(_WIN32) && !defined(_GNU_SOURCE)
+// for vasprintf()
 #define _GNU_SOURCE
+#endif
 
 #include <stdio.h>
 #include <stdlib.h>