Specify alternate compiler and linker to make - c++

Question

I have both a default C/C++ tool set (gcc 4.8) and have recently built and installed 5.2 in its own directory. I want to build some C++ programs using a makefile that uses CC and CXX explicity to compile but uses the implicit built-in rule to link the .o files to build the executable.
When I run make on the makefile, I use the command
make all CC=/usr/gcc-5.2.0/bin/gcc-5.2.0 CXX=/usr/gcc-5.2.0/bin/g++-5.2.0
The compile steps all use the 5.2 compilers but when the executable is build, the default g++ is used to link everything. Now, this happens to work and the results run but it isn't what I want. I tried adding
LD=/usr/gcc-5.2.0/bin/g++-5.2.0
to the make command line but LD is ignored. Without changing the makefile, how can I get the link step to use the 5.2 compiler?
Here is the make file:
CPPFLAGS = -Wall
LDFLAGS = -lstdc++
CXX = g++
CC = g++
all: TestSHA3 HashSHA3 HashZeroBytes LongTest sha3sum
debug: override CPPFLAGS += -ggdb
debug: all
SHA3-o3:
$(CXX) $(CPPFLAGS) -O3 -c -o SHA3.o SHA3.cpp
o3: SHA3-o3 all
TestSHA3: TestSHA3.o SHA3.o
HashSHA3: HashSHA3.o SHA3.o
HashZeroBytes: HashZeroBytes.o SHA3.o
LongTest: LongTest.o SHA3.o
sha3sum: sha3sum.o SHA3.o
.PHONY: clean realclean rc debug all o3 SHA3-o3
clean:
rm SHA3.o TestSHA3.o HashSHA3.o HashZeroBytes.o LongTest.o sha3sum.o
rc: realclean
realclean: clean
rm TestSHA3 HashSHA3 HashZeroBytes LongTest sha3sum
Here is the command:
make all CXX=g++-5.2.0 LD=g++-5.2.0
And here are the results of the make:
g++-5.2.0 -Wall -c -o TestSHA3.o TestSHA3.cpp
g++-5.2.0 -Wall -c -o SHA3.o SHA3.cpp
g++ -lstdc++ TestSHA3.o SHA3.o -o TestSHA3
g++-5.2.0 -Wall -c -o HashSHA3.o HashSHA3.cpp
g++ -lstdc++ HashSHA3.o SHA3.o -o HashSHA3
g++-5.2.0 -Wall -c -o HashZeroBytes.o HashZeroBytes.cpp
g++ -lstdc++ HashZeroBytes.o SHA3.o -o HashZeroBytes
g++-5.2.0 -Wall -c -o LongTest.o LongTest.cpp
g++ -lstdc++ LongTest.o SHA3.o -o LongTest
g++-5.2.0 -Wall -c -o sha3sum.o sha3sum.cpp
g++ -lstdc++ sha3sum.o SHA3.o -o sha3sum

Answer 1

Use:
make all CXX=g++-5.2.0 CC=g++-5.2.0
The implicit rule for linking uses CC (make manual):
n is made automatically from n.o by running the linker (usually called
ld) via the C compiler. The precise recipe used is ‘$(CC) $(LDFLAGS)
n.o $(LOADLIBES) $(LDLIBS)’.
With your Makefile and dummy source files:
$ make all CXX=`which g++`
/usr/bin/g++ -Wall -c -o TestSHA3.o TestSHA3.cpp
/usr/bin/g++ -Wall -c -o SHA3.o SHA3.cpp
g++ -lstdc++ TestSHA3.o SHA3.o -o TestSHA3
/usr/bin/g++ -Wall -c -o HashSHA3.o HashSHA3.cpp
g++ -lstdc++ HashSHA3.o SHA3.o -o HashSHA3
/usr/bin/g++ -Wall -c -o HashZeroBytes.o HashZeroBytes.cpp
g++ -lstdc++ HashZeroBytes.o SHA3.o -o HashZeroBytes
/usr/bin/g++ -Wall -c -o LongTest.o LongTest.cpp
g++ -lstdc++ LongTest.o SHA3.o -o LongTest
/usr/bin/g++ -Wall -c -o sha3sum.o sha3sum.cpp
g++ -lstdc++ sha3sum.o SHA3.o -o sha3sum
$ make all CC=`which g++`
g++ -Wall -c -o TestSHA3.o TestSHA3.cpp
g++ -Wall -c -o SHA3.o SHA3.cpp
/usr/bin/g++ -lstdc++ TestSHA3.o SHA3.o -o TestSHA3
g++ -Wall -c -o HashSHA3.o HashSHA3.cpp
/usr/bin/g++ -lstdc++ HashSHA3.o SHA3.o -o HashSHA3
g++ -Wall -c -o HashZeroBytes.o HashZeroBytes.cpp
/usr/bin/g++ -lstdc++ HashZeroBytes.o SHA3.o -o HashZeroBytes
g++ -Wall -c -o LongTest.o LongTest.cpp
/usr/bin/g++ -lstdc++ LongTest.o SHA3.o -o LongTest
g++ -Wall -c -o sha3sum.o sha3sum.cpp
/usr/bin/g++ -lstdc++ sha3sum.o SHA3.o -o sha3sum
$ make all CC=`which g++` CXX=`which g++`
/usr/bin/g++ -Wall -c -o TestSHA3.o TestSHA3.cpp
/usr/bin/g++ -Wall -c -o SHA3.o SHA3.cpp
/usr/bin/g++ -lstdc++ TestSHA3.o SHA3.o -o TestSHA3
/usr/bin/g++ -Wall -c -o HashSHA3.o HashSHA3.cpp
/usr/bin/g++ -lstdc++ HashSHA3.o SHA3.o -o HashSHA3
/usr/bin/g++ -Wall -c -o HashZeroBytes.o HashZeroBytes.cpp
/usr/bin/g++ -lstdc++ HashZeroBytes.o SHA3.o -o HashZeroBytes
/usr/bin/g++ -Wall -c -o LongTest.o LongTest.cpp
/usr/bin/g++ -lstdc++ LongTest.o SHA3.o -o LongTest
/usr/bin/g++ -Wall -c -o sha3sum.o sha3sum.cpp
/usr/bin/g++ -lstdc++ sha3sum.o SHA3.o -o sha3sum