cxx: only set GLIBCXX_DEBUG if using libstdc++

Some libraries detect the debug mode of the stdlib and try to integrate
with the system headers (eg, range-v3) which obviously doesn't work well
if we're using an alternative stdlib like clang's libc++.

nc.cmake

@@ -13,6 +13,7 @@ set (THREADS_PREFER_PTHREAD_FLAG ON)
 find_package (Threads REQUIRED)
 link_libraries (Threads::Threads)
 
+# cxx.cmake must be imported first
 include ("${CMAKE_CURRENT_LIST_DIR}/nc_cxx.cmake")
 include ("${CMAKE_CURRENT_LIST_DIR}/nc_sanitizer.cmake")
 include ("${CMAKE_CURRENT_LIST_DIR}/nc_platform.cmake")

nc_cxx.cmake

@@ -32,16 +32,19 @@ append_compile_flag ("/std:c++latest")
 
 
 ###############################################################################
-set(STDLIB "libc++" CACHE STRING "Which stdlib to specify for Clang")
+# STDLIB must be set after this block.
 
 if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+    set(STDLIB "libc++")
     append_compile_flag ("-stdlib=${STDLIB}")
     append_link_flag ("-fuse-ld=lld")
-endif ()
-
-if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
+elseif(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
+    set(STDLIB "libstdc++")
     append_link_flag ("-fuse-ld=gold")
-endif ()
+else()
+    message(FATAL_ERROR "Unknown CXX_COMPILER_ID: ${CMAKE_CXX_COMPILER_ID}")
+endif()
+
 
 ###############################################################################
 # find the gcc or clang filesystem library and append to libs if needed

nc_optimisation.cmake

@@ -192,8 +192,10 @@ if (CMAKE_BUILD_TYPE MATCHES Debug)
     add_definitions(-DENABLE_DEBUGGING)
 
     if (RUNTIME_DEBUGGING)
+        if ("${STDLIB}" STREQUAL "libstdc++")
             add_definitions(-D_GLIBCXX_DEBUG)
         endif()
+    endif()
 
     # Don't make any commits that use -O0 by default. Instead, either add an
     # argument that allows one to choose, or temporarily switch the lines