Uses correct `clang++` binary for non-standard versions

[Su3h7aM]

Define CXX after LLVM_CONFIG to use $LLVM_CONFIG --bindir to choose the correct binary from the specified version.

In my case using Fedora 40, the command for compilation using master:

clang++ src/main.cpp src/libtommath.cpp -Wno-switch -Wno-macro-redefined -Wno-unused-value '-DODIN_VERSION_RAW="dev-2024-04"' '-DGIT_SHA="58e12f0b1"' -std= c++14 -I/usr/lib64/llvm17/in
clude -std=c++17 -fno-exceptions -funwind-tables -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -L/usr/lib64/llvm17/lib -O3 -march=native -pthread -lm
-lstdc++ -ldl /usr/lib64/llvm17/lib/libLLVM-17.so '-Wl,-rpath=$ORIGIN' -o odin

However, the version of clang++ in my case is 18.1.1 which resulted in a SIGSEGV.

Using PR:

/usr/lib64/llvm17/bin/clang++ src/main.cpp src/libtommath.cpp -Wno-switch -Wno-macro-redefined -Wno-unused-value '-DODIN_VERSION_RAW="dev-2024-04"' '- DGIT_SHA="58e12f0b1"' -std= c++14 -I/usr/lib64/llvm17/in
clude -std=c++17 -fno-exceptions -funwind-tables -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -L/usr/lib64/llvm17/lib -O3 -march=native -pthread -lm
-lstdc++ -ldl /usr/lib64/llvm17/lib/libLLVM-17.so '-Wl,-rpath=$ORIGIN' -o odin

This change should work on all systems, but I only tested mine, anyone who can carry out tests on the others would help validate this PR.

---

Odin: dev-2024-04:d1f3a4974 OS: Fedora Linux 40 (KDE Plasma), Linux 6.8.7-300.fc40.x86_64 CPU: AMD Ryzen 7 5700X 8-Core Processor RAM: 32010 MiB Backend: LLVM 17.0.6