* Disable fallthrough attributes for the Intel compilers
On MacOS and Linux the Intel compilers may be identified as the
host compilers (Clang or GNU) but do not support the corresponding
compiler specific fallthrough attributes.
* Rearrange ifdef logic for excluding pre-C++17 fallthrough attributes
This puts Intel and PGI compilers into a separate group
and thus makes the intent and logic more obvious.
The `fmt` package have been available for `build2` users for several version, see: https://cppget.org/fmt
This simply add the minimum instructions for making a `build2` project depend on it.
There are other ways to do it, but they need more understanding of `build2`.
* add support for fallback_formatter in detail::write
* add ToString test into OStreamTest
to check fmt::to_string() with class that has output stream operator
* add WithOstreamOperator test into CompileTest
to check fmt::format() with FMT_COMPILE() and class that has output stream operator
* use conditional_t inside detail::write instead of 2 overloads
* Revert "add WithOstreamOperator test into CompileTest"
* remove Context from template parameters in detail::write
The intel-19 compiler warns about hidden variables:
```
/s/dev/nightly/libraries/ioss/src/fmt/format.h(2689): warning #1599: declaration hides variable "begin" (declared at line 2668)
FMT_CONSTEXPR void operator()(const Char* begin, const Char* end) {
^
detected during:
instantiation of "Context::iterator fmt::v7::vformat_to<ArgFormatter,Char,Context>(ArgFormatter::iterator, fmt::v7::basic_string_view<Char>, fmt::v7::basic_format_args<Context>, fmt::v7::detail::locale_ref) [with ArgFormatter=fmt::v7::detail::arg_formatter<fmt::v7::detail::buffer_appender<char>, char>, Char=char, Context=fmt::v7::format_context]" at line 3492
instantiation of "fmt::v7::detail::buffer_appender<Char> fmt::v7::detail::vformat_to(fmt::v7::detail::buffer<Char> &, fmt::v7::basic_string_view<Char>, fmt::v7::basic_format_args<fmt::v7::basic_format_context<fmt::v7::detail::buffer_appender<fmt::v7::type_identity_t<Char>>, fmt::v7::type_identity_t<Char>>>) [with Char=char]" at line 1413 of "/s/dev/nightly/libraries/ioss/src/fmt/format-inl.h"
/s/dev/nightly/libraries/ioss/src/fmt/format.h(2689): warning #1599: declaration hides variable "end" (declared at line 2669)
FMT_CONSTEXPR void operator()(const Char* begin, const Char* end) {
^
detected during:
instantiation of "Context::iterator fmt::v7::vformat_to<ArgFormatter,Char,Context>(ArgFormatter::iterator, fmt::v7::basic_string_view<Char>, fmt::v7::basic_format_args<Context>, fmt::v7::detail::locale_ref) [with ArgFormatter=fmt::v7::detail::arg_formatter<fmt::v7::detail::buffer_appender<char>, char>, Char=char, Context=fmt::v7::format_context]" at line 3492
instantiation of "fmt::v7::detail::buffer_appender<Char> fmt::v7::detail::vformat_to(fmt::v7::detail::buffer<Char> &, fmt::v7::basic_string_view<Char>, fmt::v7::basic_format_args<fmt::v7::basic_format_context<fmt::v7::detail::buffer_appender<fmt::v7::type_identity_t<Char>>, fmt::v7::type_identity_t<Char>>>) [with Char=char]" at line 1413 of "/s/dev/nightly/libraries/ioss/src/fmt/format-inl.h"
```
Rename the second set of variables to `pbegin` and `pend` to eliminate warning.
Our nvcc compilers (10.1.243 and 9.2.X) do not define the correct value for `FMT_USE_UDL_TEMPLATE` and then end up with an error later on in the build. Explicitly search for `__NVCC__` symbol not being defined. Might want to instead use `FMT_NVCC` or some other check, but the raw `__EDG_VERSION__` check is not working correctly for nvcc.
* include/fmt/format.h: int_writer: removed unnecessary iterator type re-declaration (prevents shadow-waringing in clang)
* include/fmt/format.h: int_writer: correctly cast signed integer to unsigned to prevent 'implicit conversion changes signedness'-warnings in clang.
Co-authored-by: Martin Wührer <martin.wuehrer@artech.at>
