Fixup header install into sass folder for autotools build

Author: saper

include/sass/context.h

@@ -4,6 +4,8 @@
 #include <stddef.h>
 #include <stdbool.h>
 #include <sass/base.h>
+#include <sass/values.h>
+#include <sass/functions.h>
 
 #ifdef __cplusplus
 extern "C" {

src/GNUmakefile.am

@@ -43,9 +43,12 @@ endif
 endif
 
 include_HEADERS = $(top_srcdir)/include/sass.h \
-                  $(top_srcdir)/include/sass2scss.h \
-                  $(top_srcdir)/include/sass/base.h \
-                  $(top_srcdir)/include/sass/values.h \
-                  $(top_srcdir)/include/sass/version.h \
-                  $(top_srcdir)/include/sass/context.h \
-                  $(top_srcdir)/include/sass/functions.h
+                  $(top_srcdir)/include/sass2scss.h
+
+sass_includedir = $(includedir)/sass
+
+sass_include_HEADERS = $(top_srcdir)/include/sass/base.h \
+                       $(top_srcdir)/include/sass/values.h \
+                       $(top_srcdir)/include/sass/version.h \
+                       $(top_srcdir)/include/sass/context.h \
+                       $(top_srcdir)/include/sass/functions.h