diff --git a/FFmpegSource2/Makefile.am b/FFmpegSource2/Makefile.am
index 89eafbde7..8a52a846c 100644
--- a/FFmpegSource2/Makefile.am
+++ b/FFmpegSource2/Makefile.am
@@ -1,6 +1,6 @@
 noinst_LIBRARIES = libffmpegsource2_aegisub.a
 
-AM_CPPFLAGS = -I../aegisub @WX_CPPFLAGS@
+AM_CPPFLAGS = -I../aegisub -D__UNIX__ @WX_CPPFLAGS@
 
 libffmpegsource2_aegisub_a_SOURCES = \
 	ffaudiosource.cpp \