fixing definition of CXX11_ABI for gcc-like

[memsharded]

Changelog: Bugfix: Fix the definition of D_GLIBCXX_USE_CXX11_ABI in gcc-like compilers for CMakeToolchain and AutotoolsToolchain. Define it only to D_GLIBCXX_USE_CXX11_ABI=0 for new compilers, assuming that the default is alread 1.
Docs: Omit

Address conan-io/conan-center-index#8389

Lets see if something breaks, need to add tests.

[memsharded]

Lets be aware that from https://gcc.gnu.org/onlinedocs/libstdc++/manual/using_dual_abi.html

Using the default configuration options for GCC the default value of the macro is 1 which causes the new ABI to be active, so to use the old ABI you must explicitly define the macro to 0 before including any library headers. (Be aware that some GNU/Linux distributions configure GCC 5 differently so that the default value of the macro is 0 and users must define it to 1 to enable the new ABI.)

So if we go this way, users from those distros will need to customize the CMakeToolchain, something like this:

def generate(self):
    tc = CMakeToolchain(self)
    tc.blocks["libcxx"].values["glibcxx"] = "1" if self.settings.libcxx == "libstdc++11" else "0"
    tc.generate()

There were requests to eliminate the default GLIBCXX_USE_CXX11_ABI=1, as it was raising some warnings (undefined in some compilers, but not errors), so it seems there is no perfect solution for 100% of users.

[lasote]

I would add a test with the generate you suggest in case someone uses a custom distro. Also, we have to document this default.

[memsharded]

I would add a test with the generate you suggest in case someone uses a custom distro. Also, we have to document this default.

I have added tests for both CMakeToolchain and AutotoolsToolchain, probably needs to be added to Meson, but I see no trace of libcxx there, so that would need more time.

I have also added a new conf tools.gnu:define_libcxx11_abi, as it seems it would be very inconvenient for users of ConanCenter recipes, if they don't want to fork and modify those recipes, for example. What do you think?

[lasote]

👍 Could be merged