Implement support for .l and .y files

[bird]

.l is flex input and spits out a .c file. While .y is for yacc/bison and spits out y.tab.c and y.tab.h (yacc) in the current directory. Since the .y files generate headers, they must be executed before any object files are made as it's not possible to predict which depends on the generated header and which doesn't. The name of the .h file should be configurable, but the default should use the input filename sans extension. Because yacc cannot control where it puts the output or what it's called, the rule has to be added to .NOTPARALLEL.

[bird]

Implemented flex in [1621].

[bird]

Postponing the yacc/bison bit till 0.1.4.

[bird]

Postponing the yacc/bison bit one more release.

[bird]

(In [2224]) kBuild: Implemented flex and yacc/bison support. Fixes #18.