Detect if `__attribute__ ((deprecated))` can be used for enum values

[krlmlr]

in ./configure, and use conditionally.

Per conda-forge/r-igraph-feedstock#56.

Or simply avoid using this in CRAN releases.

[szhorvat]

Marking enum values as deprecated is not extremely useful (not comparable to marking functions as deprecated). It was me who added this to the C core, to have a bit more security than just documenting some values as deprecated. In hindsight, we didn't get much use out of it.

So I'd suggest just defining IGRAPH_DEPRECATED_ENUMVAL to empty, since in R we don't have the fancy CMake mechanism to easily check the availability of various compiler extensions.

[szhorvat]

Removing from phoenix, since a quickfix was committed with #1205, which is good for the next release.

Leaving it open for now in case you want proper detection. If so, you can use this test code.

I changed the title to be accurate: This is not about detecting if __attribute__ ((deprecated)) is available in general—it is available as a function attribute in all GCC/Clang versions we care about. It's about whether it can be used with enum value, which is a very niche use case.

[krlmlr]

I propose to try adding a cmake call to ./configure, and to encode the relevant rules in our CMakeLists.txt . Currently:

cmake -Ssrc/vendor/cigraph -Bsrc/vendor/cigraph/build
# -- Version number from Git: v2.0.2.9007-4-g5cece5071b
# CMake Error at etc/cmake/version.cmake:66 (list):
#   list index: 1 out of range (-1, 0)
# Call Stack (most recent call first):
#   CMakeLists.txt:31 (include)
# 
# 
# CMake Error at etc/cmake/version.cmake:67 (list):
#   list index: 2 out of range (-1, 0)
# Call Stack (most recent call first):
#   CMakeLists.txt:31 (include)
# 
# 
# CMake Error at CMakeLists.txt:34 (project):
#   VERSION "v2.0.2.9007" format invalid.
# 
# 
# -- Configuring incomplete, errors occurred!

Is there a way to make version detection optional? Can/should we add our top-level CMakeLists.txt?

[krlmlr]

(If cmake is not available, we can fall back to sensible defaults.)