Fix MSVC warning.

[MarekKnapek]

Fixes problem described in #183.

Before:

c:\path\to\nob.h\how_to\001_basic_usage
>cl nob.c
Microsoft (R) C/C++ Optimizing Compiler Version 19.50.35721 for x86
Copyright (C) Microsoft Corporation.  All rights reserved.

nob.c
Microsoft (R) Incremental Linker Version 14.50.35721.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:nob.exe
nob.obj

c:\path\to\nob.h\how_to\001_basic_usage
>nob.exe
[INFO] created directory `build/`
[INFO] CMD: cl -I. -o build/hello src/hello.c
Microsoft (R) C/C++ Optimizing Compiler Version 19.50.35721 for x86
Copyright (C) Microsoft Corporation.  All rights reserved.

cl : Command line warning D9035 : option 'o' has been deprecated and will be removed in a future release
hello.c
Microsoft (R) Incremental Linker Version 14.50.35721.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:hello.exe
/out:build/hello.exe
hello.obj
[INFO] CMD: cl.exe /W4 /nologo /D_CRT_SECURE_NO_WARNINGS /Fe:build/foo /Fo:build/foo src/foo.c
foo.c

c:\path\to\nob.h\how_to\001_basic_usage
>build\hello.exe
Hello, World

c:\path\to\nob.h\how_to\001_basic_usage
>build\foo.exe
Foo, bar

c:\path\to\nob.h\how_to\001_basic_usage
>

After:

c:\path\to\nob.h\how_to\001_basic_usage
>cl nob.c
Microsoft (R) C/C++ Optimizing Compiler Version 19.50.35721 for x86
Copyright (C) Microsoft Corporation.  All rights reserved.

nob.c
Microsoft (R) Incremental Linker Version 14.50.35721.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:nob.exe
nob.obj

c:\path\to\nob.h\how_to\001_basic_usage
>nob.exe
[INFO] created directory `build/`
[INFO] CMD: cl -I. /Fe:build/hello src/hello.c
Microsoft (R) C/C++ Optimizing Compiler Version 19.50.35721 for x86
Copyright (C) Microsoft Corporation.  All rights reserved.

hello.c
Microsoft (R) Incremental Linker Version 14.50.35721.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:build/hello.exe
hello.obj
[INFO] CMD: cl.exe /W4 /nologo /D_CRT_SECURE_NO_WARNINGS /Fe:build/foo /Fo:build/foo src/foo.c
foo.c

c:\path\to\nob.h\how_to\001_basic_usage
>build\hello.exe
Hello, World

c:\path\to\nob.h\how_to\001_basic_usage
>build\foo.exe
Foo, bar

c:\path\to\nob.h\how_to\001_basic_usage
>

[rexim]

@MarekKnapek aight, looks good! Thanks!