Makefile: FreeBSD cannot do C99-or-below build

In "make DEVELOPER=YesPlease" builds, we try to help developers to
catch as many potential issues as they can by using -Wall and
turning compilation warnings into errors.  In the same spirit, we
recently started adding -std=gnu99 to their CFLAGS, so that they can
notice when they accidentally used language features beyond C99.

It however turns out that FreeBSD 13.0 mistakenly uses C11 extension
in its system header files regardless of what __STDC_VERSION__ says,
which means that the platform (unless we tweak their system headers)
cannot be used for this purpose.

It seems that -std=gnu99 is only added conditionally even in today's
config.mak.dev, so it is fine if we dropped -std=gnu99 from there.
Which means that developers on FreeBSD cannot participate in vetting
use of features beyond C99, but there are developers on other
platforms who will, so it's not too bad.

We might want a more "fundamental" fix to make the platform capable
of taking -std=gnu99, like working around the use of unconditional
C11 extension in its system header files by supplying a set of
"replacement" definitions in our header files.  We chose not to
pursue such an approach for two reasons at this point:

 (1) The fix belongs to the FreeBSD project, not this project, and
     such an upstream fix may happen hopefully in a not-too-distant
     future.

 (2) Fixing such a bug in system header files and working it around
     can lead to unexpected breakages (other parts of their system
     header files may not be expecting to see and do not work well
     with our "replacement" definitions).  This close to the final
     release of this cycle, we have no time for that.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

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