Allow overriding of host_os detection

on unknown platforms one can set `force_host_os` to pretend to be on a
supported one

data/bin/host_os

@@ -10,21 +10,27 @@
 # or submit itself to any jurisdiction.                                       #
 ###############################################################################
 
-_os=unknown
-if [ -e /etc/os-release ] ; then
-  _os=$(awk -F= '/^ID=/{gsub("[.\"]|(-.*)", "", $2); n=tolower($2)}/^VERSION_ID/{gsub("[.\"]", "", $2); v=$2}END{print n""v}' /etc/os-release)
-elif [ -e /etc/redhat-release ] ; then
-  if grep -q -E '(CentOS|Scientific Linux|Red Hat Enterprise Linux).*release 7' /etc/redhat-release ; then
-    _os=centos7
-  elif grep -q -E '(CentOS|Scientific Linux|Red Hat Enterprise Linux).*release 6' /etc/redhat-release ; then
-    _os=slc6
-  elif grep -q -E '(CentOS|Scientific Linux|Red Hat Enterprise Linux).*release 5' /etc/redhat-release ; then
-    _os=slc5
+if [ -z "${force_host_os}" ] ; then
+  _os=unknown
+  if [ -e /etc/os-release ] ; then
+    _os=$(awk -F= '/^ID=/{gsub("[.\"]|(-.*)", "", $2); n=tolower($2)}/^VERSION_ID/{gsub("[.\"]", "", $2); v=$2}END{print n""v}' /etc/os-release)
+  elif [ -e /etc/redhat-release ] ; then
+    if grep -q -E '(CentOS|Scientific Linux|Red Hat Enterprise Linux).*release 7' /etc/redhat-release ; then
+      _os=centos7
+    elif grep -q -E '(CentOS|Scientific Linux|Red Hat Enterprise Linux).*release 6' /etc/redhat-release ; then
+      _os=slc6
+    elif grep -q -E '(CentOS|Scientific Linux|Red Hat Enterprise Linux).*release 5' /etc/redhat-release ; then
+      _os=slc5
+    fi
+  elif [ -e /etc/lsb-release ] ; then
+    _os=$(awk -F= '/^DISTRIB_ID/{n=tolower($2)}/^DISTRIB_RELEASE/{gsub("\\.", "", $2); v=$2}END{print n""v}' /etc/lsb-release)
   fi
-elif [ -e /etc/lsb-release ] ; then
-  _os=$(awk -F= '/^DISTRIB_ID/{n=tolower($2)}/^DISTRIB_RELEASE/{gsub("\\.", "", $2); v=$2}END{print n""v}' /etc/lsb-release)
+else
+  (>&2 echo "warning: overriding host os detection (using ${force_host_os})")
+  _os=${force_host_os}
 fi
 
+
 # known compatibilities
 case "${_os}" in
   scientific7*|rhel7*|opensuse15*) _os=centos7 ;;