Add Clang flag

[Konstantin8105]

Add clang flag for solve next.

Prototype:

Transpile both sqlite3 files in according https://sqlite.org/howtocompile.html

c2go transpile -o=sqlite3.go "-DSQLITE_THREADSAFE=0 -DSQLITE_OMIT_LOAD_EXTENSION shell.c sqlite3.c"

Trigraph sequences #303

c2go transpile -o=file.go "-trigraphs file.c"

Allow passing include directory to clang #196

Design in #196 is not good, let's use clang native design(without space)
c2go transpile -o=file.go "-I/path/to/include file.c"

Solution

1. If user transpile many C file without specific clang flag, then design:
   c2go transpile file1.c file2.c file3.c

2. If user transpile many C file with specific clang flag, then design:
   c2go transpile "-CLANG_FLAG file1.c file2.c file3.c" (Added " around file list)

Note:

• In according to discussion Add support of 2 or more C source files. Fixes #63 #258 : if user not specify name of output file, then we can take at the base - first filename. For example : In case: "-CLANG_FLAG file1.c file2.c file3.c", First input file file1.c, so output will be file1.go.

[Konstantin8105]

In according to #258, from @elliotchance :

Can you also update https://github.com/elliotchance/c2go/blob/master/travis/test.sh#L86-L89 so that we transpile both sqlite3 files with the same command.

[Konstantin8105]

Design at the end look like that, for more detail see #337:

./c2go transpile -o=file.go  -clang-flag="-D_GNU_SOURCE" file.c