[PATCH v2 1/1] git-compat-util: add a test balloon for C99 support

The C99 standard was released in January 1999, now 22 years ago. It provides a variety of useful features, including variadic arguments for macros, declarations after statements, variable length arrays, and a wide variety of other useful features, many of which we already use.

We'd like to take advantage of these features, but we want to be cautious. As far as we know, all major compilers now support C99 or a later C standard, such as C11 or C17. POSIX has required C99 support as a requirement for the 2001 revision, so we can safely assume any POSIX system which we are interested in supporting has C99.

Even MSVC, long a holdout against modern C, now supports both C11 and C17 with an appropriate update. Moreover, even if people are using an older version of MSVC on these systems, they will generally need some implementation of the standard Unix utilities for the testsuite, and GNU coreutils, the most common option, has required C99 since 2009. Therefore, we can safely assume that a suitable version of GCC or clang is available to users even if their version of MSVC is not sufficiently capable.

Let's add a test balloon to git-compat-util.h to see if anyone is using an older compiler. We'll add a comment telling people how to enable this functionality on GCC and Clang, even though modern versions of both will automatically do the right thing, and ask people still experiencing a problem to report that to us on the list.

Note that C89 compilers don't provide the __STDC_VERSION__ macro, so we use a well-known hack of using "- 0". On compilers with this macro, it doesn't change the value, and on C89 compilers, the macro will be replaced with nothing, and our value will be 0.

We explicitly request the gnu99 style because we've traditionally taken advantage of some GCC- and clang-specific extensions when available and we'd like to retain the ability to do that, especially now that we're building with -pedantic in developer mode. Older compilers may in fact support C99 just fine, but default to C89 without an option, so the option is necessary. Allow it to be overridden by the user or config.mak.uname to ensure that people using other compilers can make it work.

Update the cmake configuration to require C11 for MSVC. We do this because this will make MSVC to use C11, since it does not explicitly support C99. We do this with a compiler options because setting the C_STANDARD option does not work in our CI on MSVC and at the moment, we don't want to require C11 for Unix compilers.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
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 Makefile                            |  2 +-
 contrib/buildsystems/CMakeLists.txt |  2 +-
 git-compat-util.h                   | 12 ++++++++++++
 3 files changed, 14 insertions(+), 2 deletions(-)