Pwm1234/rotate on open

and improve test for rotate_on_open=true

Add method to filesinks to return filename

Disable building benchmarks by default

Building the benchmarks requires google-benchmark to be installed which is
not the case for most of the build systems. The option now has a tiny
hint what the requirement is and where to get it.

closes #975

Update log_msg.h

`SPDLOG_NO_THREAD_ID` should only affect thread retrieval.

Fix #961 benchmark link error

Allow filename/line number at all levels. Add function name %! support.

Removed SPDLOG_SOURCE_MACROS_ON and SPDLOG_FUNCTION_SUFFIX.

Fix typo in file_helper.h

* "extenstion" -> "extension"

Fix handling of external fmt lib

Using an external fmt lib should cause the spdlog::spdlog target to have
a dependency on fmt lib - so that a consuming project does not need
to call find_package(fmt) and target_link_libraries(... fmt::fmt).

To this end a new cmake option SPDLOG_FMT_EXTERNAL is introduced which
makes spdlog depend on fmt lib and defines the SPDLOG_FMT_EXTERNAL macro
to avoid using the bundled fmt lib. The value of SPDLOG_FMT_EXTERNAL is
also stored in the installed spdlogConfig.cmake and if it is ON
find_dependency() is used to ensure the fmt::fmt target is imported.

Fix typos