fs/littlefs:littlefs shouldn't be used without C99 BOOL

[XinStellaris]

Summary

littlefs is a c99 project, and it malfunctions with c89. See disscusion in littlefs-project/littlefs#801.

To highlight this problem, littlefs added an assert in littlefs-project/littlefs#801.
However, it is better to expose this when compiling. This patch adds a compiling error when C99 requirement is not met.

Impact

Littlefs

Testing

Compiling okay.

[pkarashchenko]

Can we either add CFLAGS += -std=c99 if C99 is a prerequisite or apply patch after tarball download to get it c89 compatible?

[pkarashchenko]

I re-inspected the code of littlefs and see that CFLAGS += -std=c99 is a must and code has designated initialiser, variable declaration in the middle of the scope and other C99 features.

[xiaoxiang781216]

Can we either add CFLAGS += -std=c99 if C99 is a prerequisite or apply patch after tarball download to get it c89 compatible?

-std=c99 is tool chain specific, it isn't good to turn on it in the common code.

[pkarashchenko]

But the littlefs code is anyway C89 in compliant no matter of a toolchain.
I just saw that CONFIG_C99_BOOL is selected based on __STDC_VERSION__ , so I'm fine with the change now.

#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 199901L
#  define CONFIG_C99_BOOL 1
#endif

[xiaoxiang781216]

Yes, I think it's enough to warn the user if they enforce c89 accidently.

[davids5]

Telling them why and what to do about would be better.