fix: cmake -DSTATIC_LINK=ON on Linux/macOS (#1340)

fix: cmake -DSTATIC_LINK=ON on Linux/macOS (#1340)

Make linking dependent libraries as static a hard requirement when
STATIC_LINK is ON, as opposed to the previous behavior of falling back
to dynamic libraries.

A side-effect of this is that if only a dynamic zstd/hiredis is
available on a system, it will not be found, and the download and build
code will be invoked to build a static version from the internet.

Add a general case to handle Linux, macOS and similar by linking
libgcc/libstdc++ statically on gcc/clang.

Move the code into cmake/StaticLinkSupport.cmake and include it from the
main file.

Fix #1330

Signed-off-by: Rafael Kitover <rkitover@gmail.com>