opts: set `-DDUNE_MAX_TEST_CORES=${DUNECI_PARALLEL}`

But only if `${DUNECI_PARALLEL}` is set.

Reference: https://gitlab.dune-project.org/infrastructure/issues0/issues/47

base-10/opts.clang

-CMAKE_FLAGS="-DCMAKE_CXX_COMPILER=/usr/bin/clang++-4.0 -DCMAKE_C_COMPILER=/usr/bin/clang-4.0 -DCXX_MAX_STANDARD=17 -DCMAKE_CXX_FLAGS='-O2 -g -Wall -fcolor-diagnostics -ftemplate-backtrace-limit=0'"
+CMAKE_FLAGS="-DCMAKE_CXX_COMPILER=/usr/bin/clang++-4.0 -DCMAKE_C_COMPILER=/usr/bin/clang-4.0 -DCXX_MAX_STANDARD=17 -DCMAKE_CXX_FLAGS='-O2 -g -Wall -fcolor-diagnostics -ftemplate-backtrace-limit=0' ${DUNECI_PARALLEL:+-DDUNE_MAX_TEST_CORES=${DUNECI_PARALLEL}}"

base-10/opts.gcc

-CMAKE_FLAGS="-DCMAKE_CXX_COMPILER=/usr/bin/g++ -DCMAKE_C_COMPILER=/usr/bin/gcc -DCMAKE_CXX_FLAGS='-O2 -g -Wall -fdiagnostics-color=always'"
+CMAKE_FLAGS="-DCMAKE_CXX_COMPILER=/usr/bin/g++ -DCMAKE_C_COMPILER=/usr/bin/gcc -DCMAKE_CXX_FLAGS='-O2 -g -Wall -fdiagnostics-color=always' ${DUNECI_PARALLEL:+-DDUNE_MAX_TEST_CORES=${DUNECI_PARALLEL}}"

base-10/opts.gcc.c++17

-CMAKE_FLAGS="-DCMAKE_CXX_COMPILER=/usr/bin/g++-7 -DCMAKE_C_COMPILER=/usr/bin/gcc-7 -DCXX_MAX_STANDARD=17 -DCMAKE_CXX_FLAGS='-O2 -g -Wall -fdiagnostics-color=always'"
+CMAKE_FLAGS="-DCMAKE_CXX_COMPILER=/usr/bin/g++-7 -DCMAKE_C_COMPILER=/usr/bin/gcc-7 -DCXX_MAX_STANDARD=17 -DCMAKE_CXX_FLAGS='-O2 -g -Wall -fdiagnostics-color=always' ${DUNECI_PARALLEL:+-DDUNE_MAX_TEST_CORES=${DUNECI_PARALLEL}}"

base-16.04/opts.clang

-CMAKE_FLAGS="-DCMAKE_CXX_COMPILER='/usr/bin/clang++' -DCMAKE_C_COMPILER=/usr/bin/clang -DCMAKE_CXX_FLAGS='-O2 -g -Wall -fcolor-diagnostics -ftemplate-backtrace-limit=0'"
+CMAKE_FLAGS="-DCMAKE_CXX_COMPILER='/usr/bin/clang++' -DCMAKE_C_COMPILER=/usr/bin/clang -DCMAKE_CXX_FLAGS='-O2 -g -Wall -fcolor-diagnostics -ftemplate-backtrace-limit=0' ${DUNECI_PARALLEL:+-DDUNE_MAX_TEST_CORES=${DUNECI_PARALLEL}}"

base-16.04/opts.gcc

-CMAKE_FLAGS="-DCMAKE_CXX_FLAGS='-O2 -g -Wall -fdiagnostics-color=always'"
+CMAKE_FLAGS="-DCMAKE_CXX_FLAGS='-O2 -g -Wall -fdiagnostics-color=always' ${DUNECI_PARALLEL:+-DDUNE_MAX_TEST_CORES=${DUNECI_PARALLEL}}"

base-16.10/opts.clang

-CMAKE_FLAGS="-DCMAKE_CXX_COMPILER='/usr/bin/clang++' -DCMAKE_C_COMPILER=/usr/bin/clang -DCMAKE_CXX_FLAGS='-O2 -g -Wall -fcolor-diagnostics -ftemplate-backtrace-limit=0'"
+CMAKE_FLAGS="-DCMAKE_CXX_COMPILER='/usr/bin/clang++' -DCMAKE_C_COMPILER=/usr/bin/clang -DCMAKE_CXX_FLAGS='-O2 -g -Wall -fcolor-diagnostics -ftemplate-backtrace-limit=0' ${DUNECI_PARALLEL:+-DDUNE_MAX_TEST_CORES=${DUNECI_PARALLEL}}"

base-16.10/opts.gcc

-CMAKE_FLAGS="-DCMAKE_CXX_FLAGS='-O2 -g -Wall -fdiagnostics-color=always'"
+CMAKE_FLAGS="-DCMAKE_CXX_FLAGS='-O2 -g -Wall -fdiagnostics-color=always' ${DUNECI_PARALLEL:+-DDUNE_MAX_TEST_CORES=${DUNECI_PARALLEL}}"

base-8-backports/opts.clang

-CMAKE_FLAGS="-DCMAKE_CXX_COMPILER='/usr/bin/clang++-3.8' -DCMAKE_C_COMPILER=/usr/bin/clang-3.8 -DCMAKE_CXX_FLAGS='-O2 -g -Wall -fcolor-diagnostics -ftemplate-backtrace-limit=0'"
+CMAKE_FLAGS="-DCMAKE_CXX_COMPILER='/usr/bin/clang++-3.8' -DCMAKE_C_COMPILER=/usr/bin/clang-3.8 -DCMAKE_CXX_FLAGS='-O2 -g -Wall -fcolor-diagnostics -ftemplate-backtrace-limit=0' ${DUNECI_PARALLEL:+-DDUNE_MAX_TEST_CORES=${DUNECI_PARALLEL}}"

base-8/opts.clang

-CMAKE_FLAGS="-DCMAKE_CXX_COMPILER='/usr/bin/clang++' -DCMAKE_C_COMPILER=/usr/bin/clang -DCMAKE_CXX_FLAGS='-O2 -Wall -ftemplate-backtrace-limit=0'"
+CMAKE_FLAGS="-DCMAKE_CXX_COMPILER='/usr/bin/clang++' -DCMAKE_C_COMPILER=/usr/bin/clang -DCMAKE_CXX_FLAGS='-O2 -Wall -ftemplate-backtrace-limit=0' ${DUNECI_PARALLEL:+-DDUNE_MAX_TEST_CORES=${DUNECI_PARALLEL}}"

base-8/opts.gcc

-CMAKE_FLAGS="-DCMAKE_CXX_FLAGS='-O2 -g -Wall'"
+CMAKE_FLAGS="-DCMAKE_CXX_FLAGS='-O2 -g -Wall' ${DUNECI_PARALLEL:+-DDUNE_MAX_TEST_CORES=${DUNECI_PARALLEL}}"

base-9/opts.clang

-CMAKE_FLAGS="-DCMAKE_CXX_COMPILER='/usr/bin/clang++' -DCMAKE_C_COMPILER=/usr/bin/clang -DCMAKE_CXX_FLAGS='-O2 -g -Wall -fcolor-diagnostics -ftemplate-backtrace-limit=0'"
+CMAKE_FLAGS="-DCMAKE_CXX_COMPILER='/usr/bin/clang++' -DCMAKE_C_COMPILER=/usr/bin/clang -DCMAKE_CXX_FLAGS='-O2 -g -Wall -fcolor-diagnostics -ftemplate-backtrace-limit=0' ${DUNECI_PARALLEL:+-DDUNE_MAX_TEST_CORES=${DUNECI_PARALLEL}}"

base-9/opts.gcc

-CMAKE_FLAGS="-DCMAKE_CXX_FLAGS='-O2 -g -Wall -fdiagnostics-color=always'"
+CMAKE_FLAGS="-DCMAKE_CXX_FLAGS='-O2 -g -Wall -fdiagnostics-color=always' ${DUNECI_PARALLEL:+-DDUNE_MAX_TEST_CORES=${DUNECI_PARALLEL}}"