ABC's .c files are compiled as C++ via -x c++ in copts, but Bazel treats them 
  as C compilation actions, so the toolchain's cxx_flags (including             
  -stdlib=libc++) are NOT applied. Without -stdlib=libc++, clang uses the       
  system's libstdc++ headers.                                                   
                                                                                
  On gLinux, the system libstdc++ headers likely have hardened                  
  containers/assertions enabled by default (common in Google's internal
  toolchain). These headers reference __glibcxx_assert_fail. But at link time,  
  the binary links against the hermetic libc++ (not libstdc++), so the symbol is
   unresolved.

  The mismatch is:
  - Compile time: ABC uses system libstdc++ headers (no -stdlib=libc++)
  - Link time: binary links libc++.a (from hermetic LLVM), no libstdc++         
                                                                       
  The fix is to add -stdlib=libc++ to ABC's COPTS so it uses the same libc++    
  headers as everything else. Since ABC is an external dependency, let me check 
  how it's pulled in.                                                           
                                                                                
  Searched for 2 patterns, read 1 file (ctrl+o to expand)     
                                                                         
● ABC is a BCR module dependency. The quickest fix is to force -stdlib=libc++
  for ABC's compilations via .bazelrc. Since ABC's .c files use -x c++ but miss
  the toolchain's -stdlib=libc++, we need to inject it.                       
                 
  Add this to .bazelrc:                                                         
                                                                                
  build --per_file_copt=.*external/abc.*@-stdlib=libc++                         
  build --host_per_file_copt=.*external/abc.*@-stdlib=libc++                    
                                                                                
  This fixes the root cause: ABC's C-files-compiled-as-C++ will use the hermetic
   libc++ headers instead of the system's libstdc++ headers, eliminating the
  __glibcxx_assert_fail reference entirely.                                     
                             
  The .bazelrc already has a similar pattern for suppressing warnings on        
  external code (line 65-66), so this is consistent.

build --copt=-stdlib=libc++    --host_copt=-stdlib=libc++