fix: add POSIX functionality for Linux compilation

[valentynbez]

Small fix to compile binaries properly on Linux:

• defines CLOCK_MONOTONIC in ggml.c
• Closes error: 'CLOCK_MONOTONIC' undeclared #54

[prusnak]

Can you add a short comment why this is required? (I assume some functions are not recognized but not obvious which ones)

[valentynbez]

Sure!

[prusnak]

Sure!

I meant into the source code just above the #define

Btw the issue #54 mentions _POSIX_C_SOURCE=199309L; why did you chose newer standard and thus less compatibility?

[prusnak]

Small fix to compile binaries properly on Linux:

Which OS and which GCC version do you use?

For me the master compiles just fine on Debian 11 with GCC 10.2.1 (even without the proposed define).

[valentynbez]

CentOS7 with GCC 10.2.0

[ggerganov]

This flags has been discussed in whisper.cpp too and I still don't know when it should be added and when not.
Hopefully someone can clarify.

ggml-org/whisper.cpp#37
ggml-org/whisper.cpp#576

[prusnak]

Now thinking more about this, probably the cleanest option is to add compilation flags to the build system like suggested in ggml-org/whisper.cpp#37 + ggml-org/whisper.cpp#576

CFLAGS += -D_POSIX_SOURCE -D_GNU_SOURCE
CXXFLAGS += -D_POSIX_SOURCE -D_GNU_SOURCE

We can probably set this for all compilers on all OSes, because either a compiler understands the flag and sets the value to the supported level or the flag is ignored.

@valentynbez can you confirm this fixed the issue for you on CentOS 7?