clang source to source checks need to be able to pass extra flags to the compiler

[calewis]

Example of a check that fails without -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk, but as far as I can tell it isn't easy to ask make check to use this.

SSTMAC_CLANG_TEST_PREFIX=xyz SSTMAC_SKELETONIZE=1 SSTMAC_DELETE_TEMPS=0 \
  ../bin/sstmac_clang pp.pragma_sst_null_vector.cc -- -std=c++11 -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk


clang++ -std=c++11 -c sst.pp.pragma_sst_null_vector.cc -o tmp.o \
    -I.. \
    -I/sprockit -I../sprockit \
    -I/sprockit -I../sprockit \
    -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk \
    -include sstmac/compute.h -include sstmac/skeleton.h 2>&1 /dev/null | grep error >& test_clang_pragma_sst_null_vector_cpp.tmp-out