a) For `nixos-rebuild` you can set
  { nixpkgs.config.allowBroken = true; }
in configuration.nix to override this.

b) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
  { allowBroken = true; }
to ~/.config/nixpkgs/config.nix.

/nix/store/5c74da2lnbqky6pr4a73vbq36hsxg9jy-girara-0.2.9

Project name: girara
Native C compiler: /nix/store/8b0gj3bmcy5bh1zcnhn0146199b3m5kb-clang-wrapper-5.0.1/bin/cc (clang 5.0.1 "clang version 5.0.1 (tags/RELEASE_501/final)")
Build machine cpu family: x86_64
Build machine cpu: x86_64

meson.build:33:0: ERROR: C library 'libm' not found

A full log can be found at /private/tmp/nix-build-girara-0.2.9.drv-0/girara-0.2.9/build/meson-logs/meson-log.txt
builder for '/nix/store/2ba6p4bqzd0aki78q1fjb8zajjyq5x3l-girara-0.2.9.drv' failed with exit code 1
�[31;1merror:�[0m build of '/nix/store/2ba6p4bqzd0aki78q1fjb8zajjyq5x3l-girara-0.2.9.drv' failed

   Compiling proc_macro v0.0.0 (file:///build/rustc-1.25.0-src/src/libproc_macro)
   Compiling rustc_const_math v0.0.0 (file:///build/rustc-1.25.0-src/src/librustc_const_math)
   Compiling syntax_ext v0.0.0 (file:///build/rustc-1.25.0-src/src/libsyntax_ext)
building of '/nix/store/0p4h4m63ki245n14vpk2xihn23lq1a4a-rustc-1.25.0.drv' timed out after 3600 seconds
cannot build derivation '/nix/store/apcj2slll2axzpsnpyw7qxq0mymqdhly-cargo-0.26.0.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/qi8m0z84cdhfspfx4chzl5ffamky2pm6-librsvg-2.42.2.drv': 2 dependencies couldn't be built
cannot build derivation '/nix/store/51ql8gdyzkbc3mik7nqmyzxxf38by52d-djvulibre-3.5.27.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/1zjdh94j73b440qh7kabrqh6ykv6z34y-zathura-djvu-0.2.8.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/f3367gzik1vyzvmm73r0gj1y7vgpsfk3-zathura-with-plugins-0.3.9.drv': 1 dependencies couldn't be built
�[31;1merror:�[0m build of '/nix/store/f3367gzik1vyzvmm73r0gj1y7vgpsfk3-zathura-with-plugins-0.3.9.drv' failed

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