enhance set_default to support caching

Set_default is now close to CMake's set with extra powers:

• ENV will make it look for a variable with the same name in cmake calling environment
• value is used only if variable did not exist before, otherwise current value is used (allowing to override from command-line)

Add support for the following:

# declare var in CACHE as a string, set its value to 1234 (if it was not already set), override from `ENV` if required, always `FORCE`-refresh to latest value
set_default(var "1234" CACHE STRING "My var" ENV FORCE)

# set local variable to 1234 if it did not exist before
set_default(var "1234")

# set local variable var to its associated env value or default to 1234 if it's not already set
set_default(var "1234" ENV)

assigned to @gdeblasi

added 1 commit

• 3c04d5de - simplify set_default arguments handling

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approved this merge request

added 4 commits

• 63c0bbb2 - 1 commit from branch master
• 0fbbd7c1 - enhance set_default to support caching
• 18007728 - simplify set_default arguments handling
• 7d931e15 - better gcov support

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approved this merge request

mentioned in commit 91318a12

merged