gcem: new package

gcem: new package

GCE-Math (Generalized Constant Expression Math) is a templated C++
library enabling compile-time computation of mathematical functions.
It is a header-only library, so this package is BUILDONLY:=1; the
headers and CMake config files land in staging_dir for consumers to
pick up via find_package(GCEM).

Needed as a build dependency for fluidsynth >= 2.5, whose upstream
build expects gcem at configure time and (absent a system copy)
falls back to a CMake-time download from GitHub - which breaks in
offline / restricted-network build environments such as the OpenWrt
CI.

Pinned to commit 012ae73c (2024-04-28), the revision referenced by
fluidsynth 2.5.x's bundled FindGCEM.cmake / git submodule.

CMAKE_POLICY_VERSION_MINIMUM=3.5 is set because upstream's
CMakeLists.txt declares cmake_minimum_required(VERSION 3.1), which
trips current CMake's deprecated-policy guard.

Link: https://github.com/kthohr/gcem
Signed-off-by: Daniel Golle <redacted>