Check whether a previous compile succeeded before succeeding on a cached source file.
Check whether a previous compile succeeded before succeeding on a cached
source file.

fixes #99

parvmor/codeworld,stefan-j/codeworld,stefan-j/codeworld,Ye-Yong-Chi/codeworld,xwysp/codeworld,d191562687/codeworld,alphalambda/codeworld,pranjaltale16/codeworld,venkat24/codeworld,xwysp/codeworld,Ye-Yong-Chi/codeworld,nomeata/codeworld,stefan-j/codeworld,tgdavies/codeworld,google/codeworld,nomeata/codeworld,pranjaltale16/codeworld,parvmor/codeworld,pranjaltale16/codeworld,d191562687/codeworld,google/codeworld,alphalambda/codeworld,tgdavies/codeworld,google/codeworld,alphalambda/codeworld,nomeata/codeworld,d191562687/codeworld,venkat24/codeworld,three/codeworld,Ye-Yong-Chi/codeworld,tgdavies/codeworld,xwysp/codeworld,three/codeworld,three/codeworld,venkat24/codeworld,parvmor/codeworld