diff --git a/tests/libtest/Makefile.am b/tests/libtest/Makefile.am
index c50f68c64f..66c183dde2 100644
--- a/tests/libtest/Makefile.am
+++ b/tests/libtest/Makefile.am
@@ -61,7 +61,7 @@ DEPENDENCIES = $(top_builddir)/lib/libcurl.la
 
 # Mostly for Windows build targets, when using static libcurl
 if USE_CPPFLAG_CURL_STATICLIB
-AM_CPPFLAGS = -DCURL_STATICLIB
+AM_CPPFLAGS += -DCURL_STATICLIB
 endif
 
 # Makefile.inc provides the source defines (TESTUTIL, SUPPORTFILES,
diff --git a/tests/server/Makefile.am b/tests/server/Makefile.am
index 7e6db1ffe1..99cc0d67dd 100644
--- a/tests/server/Makefile.am
+++ b/tests/server/Makefile.am
@@ -51,7 +51,7 @@ AM_CPPFLAGS = -I$(top_builddir)/include/curl \
 endif
 
 if DOING_NATIVE_WINDOWS
-AM_CPPFLAGS = -DCURL_STATICLIB
+AM_CPPFLAGS += -DCURL_STATICLIB
 endif
 
 # Makefile.inc provides neat definitions