The gcc-7.2.0 compiler (and others) were giving shadowed variable warnings for this include file.  A simple renaming of a couple local variables eliminates the warnings.

Fix DLL visibility of explicit instantiation "declaration" of internal::basic_data<void> in header format.h and the explicit instantiation "definition" in format.cc (#1134)

* Update format.cc

As the explicit instantiation *declaration* of `internal::basic_data<void>` in format.h, this explicit instantiation *definition* should mirror FMT_API also.

* Mirror visibility of explicit instantiation declaration 

explicit instantiation declaration of internal::basic_data<void> should mirror visibility of FMT_API

* Eliminate `__declspec(dllexport)` designation on extern template internal::basic_data<> when `extern` affected during exporting phase.

* Add `FMT_EXTERN_TEMPLATE_API` for designate DLL export `extern template`

When exporting DLL, do not designate `__declspec(dllexport)` any template that has any explicit class template declaration a.k.a. `extern template`. Instead, designate `__declspec(dllexport)` at single point where we have explicit class template definition a.k.a. normal instantiation without `extern`

Note: this is a c++11 feature.

* Delete whole `FMT_USE_EXTERN_TEMPLATES` block and its condition

1. Remove whole `FMT_USE_EXTERN_TEMPLATES` block
(trailing `FMT_UDL_TEMPLATE` block)
````
#ifndef FMT_USE_EXTERN_TEMPLATES
#  ifndef FMT_HEADER_ONLY
#    define FMT_USE_EXTERN_TEMPLATES                           \
      ((FMT_CLANG_VERSION >= 209 && __cplusplus >= 201103L) || \
       (FMT_GCC_VERSION >= 303 && FMT_HAS_GXX_CXX11))
#  else
#    define FMT_USE_EXTERN_TEMPLATES 0
#  endif
#endif
````

2. Delete `FMT_USE_EXTERN_TEMPLATES` condition, only condition, that trailing basic_data class template definition.
````
#if FMT_USE_EXTERN_TEMPLATES
extern template struct basic_data<void>;
#endif
````

3. Replace `FMT_API` with new `FMT_EXTERN_TEMPLATE_API` added in `core.h` for sake of extern template of `basic_data<void>`

* Add `#define FMT_EXTERN extern` only when not `FMT_HEADER_ONLY`

* Replace `extern` on basic_data<void> with the `FMT_EXTERN` condition in core.h

* replace misspelled if !define() with ifndef

# Conflicts:
#	test/chrono-test.cc

fmt/include/fmt/core.h:246:
typename std::make_unsigned<_Tp>::type fmt::v5::internal::to_unsigned(Int)
[with Int = long int;
 typename std::make_unsigned<_Tp>::type = long unsigned int]:
Assertion `(value >= 0) && "negative value"' failed.

and remove redundant check in grisu2_prettify.

Change refs to fmt/time.h -> fmt/chrono.h
RST label rename time-api -> chrono-api

This reduces symbol sizes and gets rid of shadowing warnings.