[BUG] Fix warnings on OpenBSD during build with clang++

[lcheylus]

On OpenBSD current/amd64, with recent updates of libc++ (sync with LLVM 16.0.6), it's possible to build/linkg btop with clang++ compiler (version 16.0.6).

During compilation, there are some warnings:

$ gmake CXX=clang++
PLATFORM     ?| OpenBSD
ARCH         ?| amd64
GPU_SUPPORT  :| false
CXX          ?| clang++ (16.0.6)
THREADS      :| 2
REQFLAGS     !| -std=c++20
WARNFLAGS    :| -Wall -Wextra -pedantic
OPTFLAGS     :| -O2 -ftree-vectorize -flto=thin
LDCXXFLAGS   :| -pthread -D_GLIBCXX_ASSERTIONS -D_FILE_OFFSET_BITS=64 -fexceptions -fstack-clash-protection -fcf-protection -fstack-protector -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=3 -lkvm
CXXFLAGS     +| $(REQFLAGS) $(LDCXXFLAGS) $(OPTFLAGS) $(WARNFLAGS)
LDFLAGS      +| $(LDCXXFLAGS) $(OPTFLAGS) $(WARNFLAGS)
Compiling src/btop.cpp
clang++: warning: -lkvm: 'linker' input unused [-Wunused-command-line-argument]
clang++: warning: argument unused during compilation: '-fstack-clash-protection' [-Wunused-command-line-argument]
Compiling src/btop_config.cpp
clang++: warning: -lkvm: 'linker' input unused [-Wunused-command-line-argument]
clang++: warning: argument unused during compilation: '-fstack-clash-protection' [-Wunused-command-line-argument]
9%   -> obj/btop_config.o             (560KiB) (13s)
Compiling src/btop_draw.cpp
clang++: warning: -lkvm: 'linker' input unused [-Wunused-command-line-argument]
clang++: warning: argument unused during compilation: '-fstack-clash-protection' [-Wunused-command-line-argument]
18%  -> obj/btop.o                    (640KiB) (14s)
(...)

To fix these warnings, Makefile must be modified:

• remove -fstack-clash-protection flag
• add -lkvm flag only for linking, not for build

[imwints]

These warnings are really verbose and can be ignored safely. If it still bugs you, you can disable them with -Wno-unused-command-line-argument. The same thing happens on FreeBSD and macOS as well.