Allow the inclusion of an external fmtlib as as sub_directory

Shadowed parameter

However, not in bundled fmt. Corrects compiling with basic logger with header only and -Werror.

CMAKE_BINARY_DIR -> CMAKE_CURRENT_BINARY_DIR

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"