--- a/lib/libxsupgui/Makefile.in
+++ b/lib/libxsupgui/Makefile.in
@@ -91,7 +91,7 @@ install_sh = @install_sh@
 with_openssl_libs = @with_openssl_libs@
 AUTOMAKE_OPTIONS = foreign no-dependencies
 
-INCLUDES = ${top_srcdir}/src
+INCLUDES = -I${top_srcdir}/src
 
 include_HEADERS = xsupgui.h
 lib_LIBRARIES = libxsupgui.a
