The `dnl` (delete to next line) directive in this combination of `m4_if`
macros and arguments isn't properly replaced and a literal dnl string is
appended in the configure script. The `[]dnl` works ok.
ICU 59 already requires C++11 by default. The minimum version required
by the core is 50, which is compiled with at least C++11 in many distros
as package defs tell. Headers for ICU versions between ICU 50 and 58 look
fine when included for C++11 compilation, the linking is thereof not affected.
The macro PHP_CXX_COMPILE_STDCXX is based on
https://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx.html
The patch consists on allowing to read the corresponding switch in a user
defined variable instead of enforcing CXXFLAGS globally. That way, every
ext or SAPI can decide, which C++ standard is to be used. The
documentation is provided in the m4 file.
C++11 is already somewhat older standard, C++14 were better. However
issues with GCC < 5.0 and some other compilers are possibly to hit back.
Still there's some time to check for C++14 for ext/intl, too. Having said
that, C++11 in ext/intl and a mechanism to determine features is a good step
towards better C++ support.
