Minimal changes to add clang support

[valassi]

This patch fixes two build warnings (unused variables) and especially a link error for runTest.exe.

/cvmfs/sft.cern.ch/lcg/releases/clang/10.0.0-62e61/x86_64-centos7/bin/clang++  -O3  -std=c++11 -I. -I../../src -I../../../../../tools -DUSE_NVTX -Wall -Wshadow -Wextra -fopenmp  -ffast-math  -I/usr/local/cuda-11.0/include/ -c CPPProcess.cc -o CPPProcess.o
In file included from CPPProcess.cc:444:
./CPPProcess.h:89:9: warning: private field 'm_numiterations' is not used [-Wunused-private-field]
    int m_numiterations;
        ^
./CPPProcess.h:103:14: warning: private field 'amp' is not used [-Wunused-private-field]
    cxtype** amp;
             ^
2 warnings generated.
/cvmfs/sft.cern.ch/lcg/releases/clang/10.0.0-62e61/x86_64-centos7/bin/clang++ check.o -o check.exe CPPProcess.o  -O3  -std=c++11 -I. -I../../src -I../../../../../tools -DUSE_NVTX -Wall -Wshadow -Wextra -fopenmp  -ffast-math  -ldl -pthread -L../../lib -lmodel_sm -L/usr/local/cuda-11.0/lib64/ -lcuda -lcurand
/cvmfs/sft.cern.ch/lcg/releases/clang/10.0.0-62e61/x86_64-centos7/bin/clang++  -O3  -std=c++11 -I. -I../../src -I../../../../../tools -I../../../../../test/googletest/googletest/include/ -I../../../../../test/include/ -DUSE_NVTX -Wall -Wshadow -Wextra -fopenmp  -ffast-math  -I/usr/local/cuda-11.0/include/ -c runTest.cc -o runTest.o
/cvmfs/sft.cern.ch/lcg/releases/clang/10.0.0-62e61/x86_64-centos7/bin/clang++  -O3  -std=c++11 -I. -I../../src -I../../../../../tools -I../../../../../test/googletest/googletest/include/ -I../../../../../test/include/ -DUSE_NVTX -Wall -Wshadow -Wextra -fopenmp  -ffast-math  -I/usr/local/cuda-11.0/include/ -c ../../../../../test/src/MadgraphTest.cc -o ../../../../../test/src/MadgraphTest.o
/usr/local/cuda-11.0/bin/nvcc -o runTest_cu.o -c -x cu runTest.cc  -O3  -lineinfo -std=c++14 -I. -I../../src -I../../../../../tools -I../../../../../test/googletest/googletest/include/ -I../../../../../test/include/ -I/usr/local/cuda-11.0/include/ -DUSE_NVTX -arch=compute_70 -use_fast_math 
/usr/local/cuda-11.0/bin/nvcc -o runTest.exe CPPProcess.o runTest.o ../../../../../test/src/MadgraphTest.o gCPPProcess.o runTest_cu.o  -O3  -lineinfo -std=c++14 -I. -I../../src -I../../../../../tools -I../../../../../test/googletest/googletest/include/ -I../../../../../test/include/ -I/usr/local/cuda-11.0/include/ -DUSE_NVTX -arch=compute_70 -use_fast_math  -ldl -L../../lib -lmodel_sm -L../../../../../test/googletest/build/lib// -lgtest -lgtest_main -L/usr/local/cuda-11.0/lib64/ -lcuda -lcurand -lcuda -lgomp
CPPProcess.o: In function `Proc::sigmaKin(double const*, double*, int)':
CPPProcess.cc:(.text+0x29df): undefined reference to `__kmpc_fork_call'
CPPProcess.o: In function `.omp_outlined.':
CPPProcess.cc:(.text+0x2a69): undefined reference to `__kmpc_for_static_init_4'
CPPProcess.cc:(.text+0x2a93): undefined reference to `__kmpc_for_static_fini'
collect2: error: ld returned 1 exit status
make: *** [runTest.exe] Error 1

To solve the link error, the fix is to add the clang library via -L explicitly.

Initially I had also used -lomp instead of -lgomp: later on, however, I realised that -lgomp could be used also for clang, as libgomp.so is a symlink to libomp.so (in all clang installations I found on SFT cvmfs):

ls -l /cvmfs/sft.cern.ch/lcg/releases/clang/10.0.0-62e61/x86_64-centos7/lib/libgomp.so 
lrwxrwxrwx. 1 cvmfs cvmfs 9 Apr  3  2020 /cvmfs/sft.cern.ch/lcg/releases/clang/10.0.0-62e61/x86_64-centos7/lib/libgomp.so -> libomp.so*

[valassi]

Self-merging. The initial version of these fixes were in the klas2 PR #132 but I find it neater to separate the SIMD and clang issues. Note that the clang build errors in fact were already present in master.