Log Message:
spdlog: update to 1.7.0.

* Support for using external fmt version 7.x (bundled version is 6.2.1). Thanks \ 
@candrews .(22bee81).
* Fixed to_hex segmentation fault (#1611, 19f2804). Thanks @vitaut, @xvitaly, \ 
@kekePower and @lgbaldoni.
* Support for FMT_STRING compile time checking. Thanks @Tridacnid (30ee690).
* Fixed shared library building failure on Windows with non MSVC. Thanks \ 
@podsvirov (fe97a03).
* Fixed mingw testcase. Thanks @podsvirov (075dcee).
* Clean CMakeLists.txt. Thanks @Pospelove (cf55e5d).