Various MSVC-related changes.
diff --git a/Makefile.am b/Makefile.am
index 2c2444e..fcb2098 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -194,7 +194,7 @@
 
 libffi_la_LDFLAGS = -version-info `grep -v '^\#' $(srcdir)/libtool-version` $(LTLDFLAGS) $(AM_LTLDFLAGS)
 
-AM_CPPFLAGS = -I. -I$(top_srcdir)/include -Iinclude -I$(top_srcdir)/src
+AM_CPPFLAGS = -I. -I$(top_srcdir)/include -Iinclude -I$(top_srcdir)/src -DFFI_BUILDING
 AM_CCASFLAGS = $(AM_CPPFLAGS) -g
 
 # No install-html or install-pdf support in automake yet