v1.3.248 fails to build with libc++

[jbeich]

Regressed by 6dd7a3d. Found on FreeBSD but OpenBSD, Android, macOS, ChimeraLinux also use libc++ by default (OpenMandriva seems to use Clang with libstdc++). BSD (unlike GNU) enable extensions by default unless _POSIX_C_SOURCE or _XOPEN_SOURCE is defined. Unfortunately, libc++ uses some of the extensions without resetting POSIX conformance.

$ export CC=clang CXX=clang++ CXXFLAGS=-stdlib=libc++
$ cmake -DBUILD_TESTS=OFF -G Ninja -B _build
$ cmake --build _build
[...]
FAILED: layers/CMakeFiles/VkLayer_khronos_synchronization2.dir/synchronization2.cpp.o
/usr/bin/c++ -DVK_ENABLE_BETA_EXTENSIONS -DVkLayer_khronos_synchronization2_EXPORTS -D_POSIX_C_SOURCE=200112 -Ilayers -Iutils -Iutils/generated -isystem /usr/local/include -Wpointer-arith -Wno-unused-function -Wno-sign-compare -fPIC -Wall -Wextra -Wno-unused-parameter -Wno-missing-field-initializers -fno-strict-aliasing -fno-builtin-memcmp -fvisibility=hidden -std=c++17 -MD -MT layers/CMakeFiles/VkLayer_khronos_synchronization2.dir/synchronization2.cpp.o -MF layers/CMakeFiles/VkLayer_khronos_synchronization2.dir/synchronization2.cpp.o.d -o layers/CMakeFiles/VkLayer_khronos_synchronization2.dir/synchronization2.cpp.o -c layers/synchronization2.cpp
In file included from layers/synchronization2.cpp:28:
In file included from layers/synchronization2.h:29:
In file included from utils/vk_concurrent_unordered_map.h:29:
In file included from /usr/include/c++/v1/iomanip:47:
In file included from /usr/include/c++/v1/istream:165:
In file included from /usr/include/c++/v1/ostream:171:
In file included from /usr/include/c++/v1/ios:221:
/usr/include/c++/v1/__locale:680:16: error: use of undeclared identifier 'isascii'
        return isascii(__c) ? (__tab_[static_cast<int>(__c)] & __m) !=0 : false;
               ^
/usr/include/c++/v1/__locale:687:22: error: use of undeclared identifier 'isascii'
            *__vec = isascii(*__low) ? __tab_[static_cast<int>(*__low)] : 0;
                     ^
/usr/include/c++/v1/__locale:695:17: error: use of undeclared identifier 'isascii'
            if (isascii(*__low) && (__tab_[static_cast<int>(*__low)] & __m))
                ^
/usr/include/c++/v1/__locale:704:18: error: use of undeclared identifier 'isascii'
            if (!isascii(*__low) || !(__tab_[static_cast<int>(*__low)] & __m))
                 ^
4 errors generated.

Possible fixes:

• Convert -D_POSIX_C_SOURCE=200112 to -D_XOPEN_SOURCE=600
• Convert -D_POSIX_C_SOURCE=200112 to -D_GNU_SOURCE
• Make -D_POSIX_C_SOURCE=200112 local to the specific file(s)
• Drop <iomanip> and maybe more via include-what-you-use