MacOS does not use @prefix@ for python scripts

found during sanity checks for 1.9.10

#2010 (closed)

fix is simple (thx @andrei):

diff --git a/src/bin/shell/Makefile.am b/src/bin/shell/Makefile.am
index 786d4f676a..430116988a 100644
--- a/src/bin/shell/Makefile.am
+++ b/src/bin/shell/Makefile.am
@@ -1,5 +1,8 @@
 SUBDIRS = . tests
 
+PYTHON_PREFIX=@prefix@
+PYTHON_EXEC_PREFIX=@prefix@
+
 pkgpython_PYTHON = kea_conn.py kea_connector2.py kea_connector3.py
 
 sbin_SCRIPTS = kea-shell

not sure if this interferes with: --with-site-packages