Fix build failure on clang 7 with libc++

Unlike the GNU C++ STL, there's no implicit include for <array> in this
one, apparently.

CMakeLists.txt: removed spdlog subdir from library install

The library gets installed into ${CMAKE_INSTALL_LIBDIR}/spdlog, which
is for unix / linux a rather strange place, hence, put it where the linker
is more likely to find it.
Signed-off-by: Matthias Schoepfer <matthias.schoepfer@ithinx.io>

Fix windows compile error where std::max is overriden by a macro (#1251)

Set additional CPACK variables for RPM generator

Sets the following when querying the generated RPM's info:

Group       : System Environment/Libraries
License     : MIT
URL         : https://github.com/gabime/spdlog
Summary     : Fast C++ logging library
Description :
Very fast, header-only/compiled, C++ logging library.

cmake: Only install fmt headers when SPDLOG_FMT_EXTERNAL is not defined.

cmake: Add a new spdlog.pc pkgconfig file.

Fix #1239, Remove basic_string_view_t

Fixes https://github.com/gabime/spdlog/issues/1237.