Build fails if pkg-config is not installed

[Volker-Weissmann]

Before running pacman -S pkg-config, I got the error below.
Is it possible to improve this error message?

$ git log --oneline -n 1
b65a317 (HEAD -> master, tag: v5.1.0, origin/master, origin/auto, origin/HEAD) Auto merge of #60 - kchibisov:bump-version, r=jdm
$ cargo build
   Updating crates.io index
   Compiling pkg-config v0.3.22
   Compiling cc v1.0.72
   Compiling libc v0.2.107
   Compiling cmake v0.1.46
   Compiling servo-fontconfig-sys v5.1.0 (/home/weissmann/cloned/libfontconfig)
   Compiling expat-sys v2.1.6
   Compiling freetype-sys v0.13.1
error: failed to run custom build command for `servo-fontconfig-sys v5.1.0 (/home/weissmann/cloned/libfontconfig)`

Caused by:
  process didn't exit successfully: `/home/weissmann/cloned/libfontconfig/target/debug/build/servo-fontconfig-sys-beb914baa2d86c51/build-script-build` (exit status: 101)
  --- stdout
...
  checking for FREETYPE... no

  --- stderr
  configure: error: in `/home/weissmann/cloned/libfontconfig/target/debug/build/servo-fontconfig-sys-24f58006778fe73e/out':
  configure: error: The pkg-config script could not be found or is too old.  Make sure it
  is in your PATH or set the PKG_CONFIG environment variable to the full
  path to pkg-config.

  Alternatively, you may set the environment variables FREETYPE_CFLAGS
  and FREETYPE_LIBS to avoid the need to call pkg-config.
  See the pkg-config man page for more details.

  To get pkg-config, see <http://pkg-config.freedesktop.org/>.
  See `config.log' for more details
  make: *** [makefile.cargo:83: /home/weissmann/cloned/libfontconfig/target/debug/build/servo-fontconfig-sys-24f58006778fe73e/out/Makefile] Error 1
  thread 'main' panicked at 'assertion failed: Command::new(\"make\").env(\"MAKEFLAGS\",\n                         env::var(\"CARGO_MAKEFLAGS\").unwrap_or_default()).args(&[\"-R\",\n                                                                                 \"-f\",\n                                                                                 \"makefile.cargo\"]).status().unwrap().success()', build.rs:41:5
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace