meson: Don't use c_std=c99

meson: Don't use c_std=c99

Just use the default c_std used by the compiler. With GCC on Linux this
is gnu89.

Tons of errors related to time.h, signal.h, etc when using c99:

FAILED: libs/gst/check/libcheck/check@sta/check_run.c.o
cc  '-Ilibs/gst/check/libcheck/check@sta' '-fdiagnostics-color=always' '-I../libs/gst/check/libcheck' '-Ilibs/gst/check/libcheck' '-I.' '-I../.' '-Ilibs/gst/check/libcheck/..' '-I../libs/gst/check/libcheck/..' '-pipe' '-Wall' '-Winvalid-pch' '-std=c99' '-DHAVE_CONFIG_H' '-fPIC' '-O2' '-g' '-fPIC' '-MMD' '-MQ' 'libs/gst/check/libcheck/check@sta/check_run.c.o' '-MF' 'libs/gst/check/libcheck/check@sta/check_run.c.o.d' -o 'libs/gst/check/libcheck/check@sta/check_run.c.o' -c ../libs/gst/check/libcheck/check_run.c
In file included from ../libs/gst/check/libcheck/check_run.c:21:0:
../libs/gst/check/libcheck/libcompat.h:167:18: warning: ‘struct itimerspec’ declared inside parameter list will not be visible outside of this definition or declaration
     const struct itimerspec *new_value, struct itimerspec *old_value);
                  ^~~~~~~~~~
../libs/gst/check/libcheck/check_run.c:98:25: error: array type has incomplete element type ‘struct sigaction’
 static struct sigaction old_action[3];
                         ^~~~~~~~~~
../libs/gst/check/libcheck/check_run.c:99:25: error: array type has incomplete element type ‘struct sigaction’
 static struct sigaction new_action[3];
                         ^~~~~~~~~~

[...]

ninja: build stopped: subcommand failed.

The change was originally made because gnu99 was causing issues on OS X.