fix libnsgif build

"make dist" was picking up .o files in libnsgif/

libvips/foreign/Makefile.am

@@ -1,6 +1,4 @@
-if ENABLE_NSGIF
 SUBDIRS = libnsgif 
-endif
 
 noinst_LTLIBRARIES = libforeign.la
 
@@ -94,7 +92,5 @@ if !OPENSLIDE_MODULE
 libforeign_la_SOURCES += openslide2vips.c
 endif # !OPENSLIDE_MODULE
 
-EXTRA_DIST = libnsgif
-
 AM_CPPFLAGS = -I${top_srcdir}/libvips/include @VIPS_CFLAGS@ @VIPS_INCLUDES@
 

libvips/foreign/libnsgif/Makefile.am

@@ -1,3 +1,15 @@
+noinst_LTLIBRARIES = libnsgif.la
+
+MY_SOURCES = \
+	libnsgif.h \
+	libnsgif.c \
+	lzw.c \
+	lzw.h
+
+if ENABLE_NSGIF
+libnsgif_la_SOURCES = $(MY_SOURCES)
+endif
+
 EXTRA_DIST = \
 	README-ns \
 	README.md \
@@ -5,11 +17,7 @@ EXTRA_DIST = \
 	update.sh \
 	utils 
 
-noinst_LTLIBRARIES = libnsgif.la
-
-libnsgif_la_SOURCES = \
-	libnsgif.h \
-	libnsgif.c \
-	lzw.c \
-	lzw.h
-
+if !ENABLE_NSGIF
+EXTRA_DIST += \
+	$(MY_SOURCES) 
+endif