Allow to overwrite ABCEXTERNAL from the environment.

In this way Debian scripts can define it as berkeley-abc from the shell.

diff --git a/Makefile b/Makefile
index 4534d619ec25f20bdaa3c3028a3a630fb291e163..9f16a4fb7b97bf9048deda3b9e4c3bc8331346b6 100644 (file)
--- a/Makefile
+++ b/Makefile
@@ -90,7 +90,7 @@ ABCMKARGS = CC="$(CXX)" CXX="$(CXX)"
 
 # set ABCEXTERNAL = <abc-command> to use an external ABC instance
 # Note: The in-tree ABC (yosys-abc) will not be installed when ABCEXTERNAL is set.
-ABCEXTERNAL =
+ABCEXTERNAL ?=
 
 define newline