Use Debian codename for ISSUE_RELEASE if no version number

[d-torrance]

This is an issue in Debian testing and unstable, where lsb_release -s --release returns the uninformative "n/a". This causes an extra problem because we end up with a little "a" subdirectory under usr-dist during build.

So for example, now we get "Debian-trixie" as the issue.

[mahrud]

By the way, this would be the corresponding fix in cmake:

diff --git a/M2/cmake/flavor.cmake b/M2/cmake/flavor.cmake
index 28eab2894e..8201fab4d8 100644
--- a/M2/cmake/flavor.cmake
+++ b/M2/cmake/flavor.cmake
@@ -26,6 +26,11 @@ if(NOT DEFINED ISSUE)
     execute_process(COMMAND ${LSB_RELEASE} -s --release
       OUTPUT_VARIABLE ISSUE_RELEASE
       ERROR_QUIET OUTPUT_STRIP_TRAILING_WHITESPACE)
+    if(ISSUE_RELEASE STREQUAL "n/a")
+      execute_process(COMMAND ${LSB_RELEASE} -s --codename
+       OUTPUT_VARIABLE ISSUE_RELEASE
+       ERROR_QUIET OUTPUT_STRIP_TRAILING_WHITESPACE)
+    endif()
   elseif(OS_RELEASE)
     file(READ ${OS_RELEASE} _os_release)
     string(REGEX MATCH "ID=([A-Za-z ]*)\n"      _ "${_os_release}")