* Avoid a space in the UDL definition except on GCC before 4.9
Clang 18 has grown a warning about the space being deprecated which
is enabled by default in their nightly binaries. However GCC before 4.9
will reject the UDL definition unless there is a space there, so we need
to keep the space conditionally for it.
* Remove UDLs on GCC before 4.9 to simplify things
GCC before 4.9 rejects the syntax that is now
rejected on more modern compilers.
* Disable compile-error-test on GCC < 4.9
This avoids the UDL tests failing as GCC < 4.9 can not parse UDLs
without a space, but the space is malformed in modern compilers.
* use the standard `test-main.cc` component instead of injected test infrastructure sources
* undo now obsolete commit `00235d8a` from July 2021
* Clang cannot import user-defined literals as it seems -> disable test
* Clang emits duplicate, non-mergeable copies of `detail::buffer`'s vtable, causing linker errors -> disable test
It disables entire branch of tests declaration unconditionally because CXX_STANDARD
is not defined there. But even we use CMAKE_CXX_STANDARD here, these tests should
not be disabled with standard >= C++20.
* do this in _some_ tests to improve test coverage and catch possible problems due to that
* fix invalid export of `static constexpr` constant
* fix msvc warnings about unreachable code in high warning levels
* all GTest/GMock files moved to `test/gtest` directory
* `CMakeLists.txt` created in `test/gtest` from `CMakeLists.txt` in `test`
* GTest/GMock target in CMake renamed to `gtest` (was `gmock`)
* CMake `gtest` target updated to export includes as "gtest/gtest.h" or "gmock/gmock.h" only
* includes in tests updated: "gtest.h" -> "gtest/gtest.h", "gmock.h" -> "gmock/gmock.h"
* removed duplications of `target_include_directories` for GTest/GMock directories (CMake manages them)
* find-package-test fixed by passing CXX_FLAGS, i.e. -stdlib=libc++
* std::array usage in compile-test.cc replaced with plain array, because
<array> header was not included
- Fix C++17 Visual Studio builds.
- Use C++17 for `windows-2019` builds.
- Removed options for unsupported compiler (MSVC11).
- Removed options, not needed after enabling C++11+ features in gmock/gtest.
