Remove dependency on GNU's libc
Can't build on OpenBSD.
mustach.c:137:12: warning: implicit declaration of function 'strndupa' is invalid in C99 [-Wimplicit-function-declaration]
opstr = strndupa(beg, l);
^
mustach.c:137:10: warning: incompatible integer to pointer conversion assigning to 'const char *' from 'int'
[-Wint-conversion]
opstr = strndupa(beg, l);
^ ~~~~~~~~~~~~~~~~
mustach.c:141:10: warning: incompatible integer to pointer conversion assigning to 'const char *' from 'int'
[-Wint-conversion]
clstr = strndupa(beg + l, len - l);
^ ~~~~~~~~~~~~~~~~~~~~~~~~~~