fix build on windows

only include source files in make dist for libnsgif and module/

we were just including the whole directory, which stopped the derived
files (eg. the .la archives) from being rebuilt on other platforms

configure.ac

@@ -1494,6 +1494,7 @@ AC_CONFIG_FILES([
         libvips/histogram/Makefile
         libvips/draw/Makefile
         libvips/iofuncs/Makefile
+        libvips/module/Makefile
         libvips/morphology/Makefile
         libvips/mosaicing/Makefile
         libvips/create/Makefile

libvips/Makefile.am

@@ -27,6 +27,7 @@ SUBDIRS = \
 	draw \
 	iofuncs \
 	morphology \
+	module \
 	mosaicing \
 	create
 
@@ -61,7 +62,6 @@ libvips_la_LDFLAGS = \
 	-version-info @LIBRARY_CURRENT@:@LIBRARY_REVISION@:@LIBRARY_AGE@ 
 
 EXTRA_DIST = \
-	module \
 	$(OPTIONAL_DIST_DIR)
 
 CLEANFILES = 

libvips/module/Makefile.am

@@ -0,0 +1,5 @@
+EXTRA_DIST = \
+	heif.c \
+	magick.c \
+	openslide.c \
+	poppler.c