how do I change compilation flags for selected targets?

Dear Listreader,

how do I change compilations flags for a selected target?

In my example I have this receipe:

----------------------------------
SOURCES =·
    main.cpp
    ext.cpp

OPTIMIZE=1

all: hello_world

:program hello_world : $SOURCES
----------------------------------

now I want to compile main.cpp with -O1 (which is OPTIMIZE=1) and
ext.cpp with -O2 (which is OPTIMIZE=2). 

How can I do this? What would be the easiest / cleanest receipe
for my example?

    thanks
    Joerg



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