m4: crypt_xxpg6.m4: Define _DEFAULT_SOURCE for current glibc

[fweimer-rh]

Current glibc no longer implements the CRYPT extension, so it does not declare crypt in <unistd.h> in strict standard modes. The check defines _XOPEN_SOURCE, which enables one of these modes. Defining _DEFAULT_SOURCE as well again makes available the crypt function prototype.

This avoids a configure check result change with compilers which do not support implicit function declarations.

Related to:

• https://fedoraproject.org/wiki/Changes/PortingToModernC
• https://fedoraproject.org/wiki/Toolchain/PortingToModernC

[cmouse]

Can you provide more information about what kind of thing this fixes?

[fweimer-rh]

Without this change, building dovecot with a C99 compiler results in

checking if we should use _XPG6 macro for crypt()... no

while a compiler which supports C89-style implicit function declarations produces

checking if we should use _XPG6 macro for crypt()... yes

with the same system header files. Implicit function declarations were removed from the C language with C99, but current compilers merely report a warning and can still produce an executable if a symbol of that name is available at link time. Does this answer your question?

I don't know what the change in the configure check result means for the dovecot build.

[sirainen]

Merged as dd691d4