$ gcc -std=c99 tools/updaterevision/updaterevision.c
tools/updaterevision/updaterevision.c: In function ‘main’:
tools/updaterevision/updaterevision.c:65:11: warning: implicit declaration of function ‘popen’; did 
you mean ‘fopen’? [-Wimplicit-function-declaration]
  stream = popen("hg log --template \"{latesttag}-{latesttagdistance}-{node|short}\\n\" 
--rev . && hg log -r. --template \"{date|isodatesec}*{node}?{date|hgdate}\\n\" && 
hg identify -n", "r");
           ^~~~~
           fopen
tools/updaterevision/updaterevision.c:65:9: warning: assignment to ‘FILE *’ {aka ‘struct _IO_FILE 
*’} from ‘int’ makes pointer from integer without a cast [-Wint-conversion]
  stream = popen("hg log --template \"{latesttag}-{latesttagdistance}-{node|short}\\n\" 
--rev . && hg log -r. --template \"{date|isodatesec}*{node}?{date|hgdate}\\n\" && 
hg identify -n", "r");
         ^
tools/updaterevision/updaterevision.c:92:3: warning: implicit declaration of function ‘pclose’; did 
you mean ‘fclose’? [-Wimplicit-function-declaration]
   pclose(stream);
   ^~~~~~
   fclose

1. libc6 2.27 (glibc)
   
2. gcc-7 7.3.0 (GCC)