build: make CXX overridable (CXX ?= g++) for clang builds

The Makefile hardcoded `CXX = g++`, which a `CXX=clang++ make` environment
prefix does not override. Downstream (ProxySQL deps build) compiles this with
clang in containers that ship only clang/clang++ (no g++), so the build failed
with exit 127 (g++: command not found). Using `?=` lets the environment-provided
compiler win while keeping g++ as the default.

Author: renecannao

Makefile

@@ -1,4 +1,4 @@
-CXX = g++
+CXX ?= g++
 CXXFLAGS = -std=c++17 -Wall -Wextra -g -O2
 CPPFLAGS = -I./include -I./third_party/googletest/googletest/include