Fix building

[dpino]

I was unable to run the Makefile as it is now in master. For instance, the path to dynasm in Makefile misses third_party/ route:

$ make
lua dynasm/dynasm.lua jit2.dasc > jit2.h
lua: cannot open dynasm/dynasm.lua: No such file or directory
Makefile:11: recipe for target 'jit2.h' failed
make: *** [jit2.h] Error 1

After fixing that, I got the following error:

$ make
cc -O3 -g -std=gnu99 -Ithird_party  -o jit2 dynasm-driver.c -DJIT=\"jit2.h\"
/usr/lib/gcc/x86_64-linux-gnu/6/../../../x86_64-linux-gnu/Scrt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status
Makefile:9: recipe for target 'jit2' failed
make: *** [jit2] Error 1

The problem is that *.dasc files are compiled to .h, but they contain the main function. To fix that I turned
dynasm-driver.c into a header and compile the *.dasc files to *.c

After those changes, the Makefile can build the executables now:

$ make
cc -O3 -g -std=gnu99 -Ithird_party -D DASM_VERSION=10300    jit1.c   -o jit1
cc -O3 -g -std=gnu99 -Ithird_party -D DASM_VERSION=10300  dynasm-driver.h jit2.c -o jit2
cc -O3 -g -std=gnu99 -Ithird_party -D DASM_VERSION=10300  dynasm-driver.h jit3.c -o jit3