lang/opensycl: Fix warning messages with make describe

% pwd
/usr/ports/lang/opensycl
% make describe
sh: /usr/local/llvm15/bin/clang: not found
make: "/usr/ports/Mk/Uses/compiler.mk" line 83: warning: "/usr/local/llvm15/bin/clang --version" returned non-zero status
make: "/usr/ports/Mk/Uses/compiler.mk" line 131: warning: "/usr/local/llvm15/bin/clang++ -### /dev/null 2>&1" returned non-zero status
opensycl-0.9.4|/usr/ports/lang/opensycl|/usr/local|Multi-backend implementation of SYCL for CPUs and GPUs|/usr/ports/lang/opensycl/pkg-descr|yuri@FreeBSD.org|lang||||/usr/ports/devel/boost-libs /usr/ports/devel/cmake-core /usr/ports/devel/llvm15 /usr/ports/devel/ninja /usr/ports/lang/python311|/usr/ports/lang/python311|https://github.com/OpenSYCL/OpenSYCL
%

Approved by: portmgr (implicit, just fix it)
Fixes: 230ffdc7852d lang/opensycl: fix build without libomp