Handle correctly situation where /etc/os-release is incomplete (#75)

If the parsing of /etc/os-release failed to set either HSF_OS_ID or
HSF_OS_VERSION there was then a nasty CMake error thrown
in a later REGEX REPLACE. At least for bleeding edge releases
the VERSION_ID string can be absent (noticed on Gentoo portage)
so this broke.

In these cases we set the strings to some value that's consistent with
not knowing the correct value ("unknown v0").

cmake/prmonCPack.cmake

@@ -87,11 +87,19 @@ function(hsf_get_platform _output_var)
       # http://www.freedesktop.org/software/systemd/man/os-release.html
       # - ID
       file(STRINGS "/etc/os-release" HSF_OS_ID REGEX "^ID=.*$")
-      string(REGEX REPLACE "ID=|\"" "" HSF_OS_ID ${HSF_OS_ID})
+      if(HSF_OS_ID)
+        string(REGEX REPLACE "ID=|\"" "" HSF_OS_ID ${HSF_OS_ID})
+      else()
+        set(HSF_OS_ID "unknown")
+      endif()
       # - VERSION_ID
       file(STRINGS "/etc/os-release" HSF_OS_VERSION REGEX "^VERSION_ID=.*$")
-      string(REGEX REPLACE "VERSION_ID=|\"" "" HSF_OS_VERSION ${HSF_OS_VERSION})
-      string(REGEX REPLACE "([a-z0-9]+)(\.|-|_)([a-z0-9]+).*" "\\1\\3" HSF_OS_VERSION ${HSF_OS_VERSION})
+      if(HSF_OS_VERSION)
+        string(REGEX REPLACE "VERSION_ID=|\"" "" HSF_OS_VERSION ${HSF_OS_VERSION})
+        string(REGEX REPLACE "([a-z0-9]+)(\.|-|_)([a-z0-9]+).*" "\\1\\3" HSF_OS_VERSION ${HSF_OS_VERSION})
+      else()
+        set(HSF_OS_VERSION "0")
+      endif()
     else()
       # Workaround for older systems
       # 1. Might be lucky and have lsb_release