respect user-set compiler name

what if my clang compiler is not clang but clang-3.5?

bench/Makefile

-CXX	= g++
+CXX	?= g++
 CXXFLAGS	= -march=native -Wall -Wextra -pedantic -std=c++11 -pthread -Wl,--no-as-needed  -I../include 
 CXX_RELEASE_FLAGS = -O3 -flto
 

bench/Makefile.mingw

-CXX	= g++
+CXX	?= g++
 CXXFLAGS	= -D_WIN32_WINNT=0x600 -march=native -Wall -Wextra -pedantic -std=c++11 -pthread -Wl,--no-as-needed  -I../include 
 CXX_RELEASE_FLAGS = -O3 -flto
 

example/Makefile

-CXX	= g++
+CXX	?= g++
 CXXFLAGS	= -march=native -Wall -Wshadow -Wextra -pedantic -std=c++11 -pthread -Wl,--no-as-needed  -I../include 
 CXX_RELEASE_FLAGS = -O3 -flto
 CXX_DEBUG_FLAGS= -g 

example/Makefile.clang

-CXX	= clang++
+CXX	?= clang++
 CXXFLAGS	= -march=native -Wall -Wextra -Wshadow -pedantic -std=c++11 -pthread -I../include
 CXX_RELEASE_FLAGS = -O2
 CXX_DEBUG_FLAGS= -g 

example/Makefile.mingw

-CXX	= g++
+CXX	?= g++
 CXXFLAGS	=  -D_WIN32_WINNT=0x600 -march=native -Wall -Wextra -Wshadow -pedantic -std=c++11 -pthread -Wl,--no-as-needed  -I../include 
 CXX_RELEASE_FLAGS = -O3 
 CXX_DEBUG_FLAGS= -g