ROHD-2434: Bug fix for lofar-base code optimisation

Fix a bug where parts of the image are always compiled with
-march=haswell setting.  This breaks even the compilation of lofar-base
further down the line when executed on a non-haswell build host.

Docker/lofar-base/Dockerfile.tmpl

@@ -62,7 +62,20 @@ ENV CASACORE_VERSION=v3.1.0 \
 # Allow to specify the number of cpus as --build-arg.
 #
 ARG J=6
-ENV J=${J} CXX_FLAGS="--std=c++11 -W -Wall -Woverloaded-virtual -Wno-unknown-pragmas -D_GLIBCXX_USE_CXX11_ABI=${CXX_ABI} -O3 -march=haswell"
+ENV J=${J}
+
+# Allow to overwrite the default CXX_FLAGS settings for code compilation
+# by specifying --build-args CXX_FLAGS="blah blah"
+ARG CXX_FLAGS="--std=c++11 -W -Wall -Woverloaded-virtual -Wno-unknown-pragmas -D_GLIBCXX_USE_CXX11_ABI=${CXX_ABI} -O3"
+
+# Allow to overwrite the CPU optimisation default setting by specifying
+# --build-arg CPU_OPTIMISATION="-march=native"
+ARG CPU_OPTIMISATION="-march=haswell"
+ENV CPU_OPTIMISATION=${CPU_OPTIMISATION}
+
+# Combine CXX_FLAGS and CPU_OPTIMISATION
+ENV CXX_FLAGS=${CXXFLAGS} ${CPU_OPTIMISATION}
+
 
 #
 # Base and runtime dependencies