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>
