Allow jobs to pass additional compiler flags

Jobs can pass these additional flags in the variables `DUNECI_CFLAGS` and
`DUNECI_CXXFLAGS`.  This is similar to what is already possible with
`DUNECI_CMAKE_FLAGS`, except that that variable cannot be used to pass
additional compiler flags.

This is meant for flags that select particular sub-achitectures such as
`-march`.  Jobs still need to make sure to select a suitable runner actually
supporting those architectures via runner tags.

Addresses: core/dune-common#165

base-common/dune.opts

@@ -38,8 +38,8 @@ source ~/toolchains/${toolchain}
 CMAKE_FLAGS=""
 CMAKE_FLAGS+=" -DCMAKE_C_COMPILER=$CC"
 CMAKE_FLAGS+=" -DCMAKE_CXX_COMPILER=$CXX"
-CMAKE_FLAGS+=" -DCMAKE_C_FLAGS='${CFLAGS}'"
-CMAKE_FLAGS+=" -DCMAKE_CXX_FLAGS='${CXXFLAGS}'"
+CMAKE_FLAGS+=" -DCMAKE_C_FLAGS='${CFLAGS} ${DUNECI_CFLAGS:-}'"
+CMAKE_FLAGS+=" -DCMAKE_CXX_FLAGS='${CXXFLAGS} ${DUNECI_CXXFLAGS:-}'"
 CMAKE_FLAGS+=" -DCMAKE_BUILD_TYPE=Release"
 CMAKE_FLAGS+=" -DCMAKE_C_FLAGS_RELEASE='${REL_CFLAGS}'"
 CMAKE_FLAGS+=" -DCMAKE_CXX_FLAGS_RELEASE='${REL_CXXFLAGS}'"