CLI: `-D` flag handled differently than gcc/clang

[ehaas]

From the GCC docs for -D name=definition:

The contents of definition are tokenized and processed as if they appeared during translation phase three in a ‘#define’ directive. In particular, the definition is truncated by embedded newline characters.`

So newline splicing should not happen for -D definitions.

For a program that contains the single token FOO, clang produces the following output (gcc is the same but does not issue a warning):

➜  ~ clang -E -DFOO="F\\
OO" test.c

warning: macro 'FOO' contains embedded newline; text after the newline is ignored
F\
1 warning generated.

[ehaas]

I think we could implement this by just truncating the argument at the first newline before writing it to macro_buf in main.zig:parseArgs

[Vexu]

I think we could implement this by just truncating the argument at the first newline before writing it to macro_buf in main.zig:parseArgs

Sounds good to me.