ENH: Add option to allow to always trigger otb build step from superbuild tree
I've seen lot of new OTB developers who use the superbuild to create their environment of development for OTB. Indeed Superbuild is quite useful as it allows to build all the dependencies and as a last step build OTB (with potentially modifications from the developer).
Superbuild is pretty handy but on the other hand steps behind the main 'make' command are a bit hidden behind the cmake magic as it runs external_project macro for each sub-projects which can download/patch/configure/build/install...
One problem from some new developers is the fact that when they modify OTB sources and run make again from superbuild directory nothing happen and modify files in OTB source tree are not recompile by the superbuild.
This MR "just" add a convenient "BUILD_ALWAYS" option at the superbuild root which allows to always trigger OTB compilation at the superbuild level.
It should allow developers to compile their modifications without having to dive into superbuild build tree (OTB/build) and control the OTB build process from the root directory.
The BUILD_ALWAYS option is available since at least cmake 3.2 and cmake min version for superbuild is 3.3.
BUILD_ALWAYS is ON by default in the MR. Please reviewer let me know if we should change this.
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