diff --git a/builds/unix/configure.ac b/builds/unix/configure.ac
index e7ac82a..78088b7 100644
--- a/builds/unix/configure.ac
+++ b/builds/unix/configure.ac
@@ -70,9 +70,9 @@
 dnl Here we check whether we can use our mmap file component.
 AC_FUNC_MMAP
 if test "$ac_cv_func_mmap_fixed_mapped" != yes; then
-  FTSYS_SRC='$(BASE_)ftsystem.c'
+  FTSYS_SRC='$(BASE_DIR)/ftsystem.c'
 else
-  FTSYS_SRC='$(BUILD)/ftsystem.c'
+  FTSYS_SRC='$(BUILD_DIR)/ftsystem.c'
 
   FT_MUNMAP_DECL
   FT_MUNMAP_PARAM