cryptic "O: command not found" diagnostic in case rustc isn't installed

[dckc]

I looked at README.md and skipped right to the "Building" section with git clone, configure etc. It happily built mozjs and such, then fell over and complained about "O: command not found." Details below.

Perhaps configure should have complained earlier?

To reproduce:

1. On an Ubuntu machine, clone rust (57b4d10ff652d3beddae64782c882a07822bac3c)
2. Hastily skip past all the blah-blah in README.me and slavishly follow the directions in the Prerequisites and Building sections:
   • sudo apt-get install libcairo2-dev libpango1.0-dev autoconf2.13 freeglut3-dev
   • git clone git://github.com/mozilla/servo.git
   • cd servo
   • mkdir -p build && cd build
   • ../configure
   • make check-servo && make

Expected: configure complains about missing rustc

Actual:

connolly@pav:~/src/servo/build$ make check-servo && make
...
make[1]: Entering directory `/usr/local/src/servo/build/src/rust-mozjs'
cc  -I../mozjs/dist/include -fPIC /home/connolly/src/servo/src/rust-mozjs/jsglue.c -o jsglue.o -c
ar rcs libjsglue.a jsglue.o
O -L /home/connolly/src/servo/build/src/mozjs /home/connolly/src/servo/src/rust-mozjs/js.rc -o libjs.dummy
make[1]: O: Command not found
make[1]: [libjs.dummy] Error 127 (ignored)
touch libjs.dummy
make[1]: Leaving directory `/usr/local/src/servo/build/src/rust-mozjs'
O -L /home/connolly/src/servo/build/src/libwapcaplet -L /home/connolly/src/servo/build/src/rust-harfbuzz -L /home/connolly/src/servo/build/src/rust-opengles -L /home/connolly/src/servo/build/src/rust-azure -L /home/connolly/src/servo/build/src/rust-stb-image -L /home/connolly/src/servo/build/src/rust-geom -L /home/connolly/src/servo/build/src/rust-glut -L /home/connolly/src/servo/build/src/rust-layers -L /home/connolly/src/servo/build/src/rust-http-client -L /home/connolly/src/servo/build/src/libparserutils -L /home/connolly/src/servo/build/src/libhubbub -L /home/connolly/src/servo/build/src/libcss -L /home/connolly/src/servo/build/src/rust-netsurfcss -L /home/connolly/src/servo/build/src/servo-sandbox -L /home/connolly/src/servo/build/src/rust-hubbub -L /home/connolly/src/servo/build/src/sharegl -L /home/connolly/src/servo/build/src/rust-mozjs -L /home/connolly/src/servo/build/src/mozjs --test -o servo-test /home/connolly/src/servo/src/servo/servo.rc
make: O: Command not found
make: [servo-test] Error 127 (ignored)
./servo-test 
make: ./servo-test: Command not found
make: *** [check-servo] Error 127