Freemine.cmake.external.proj

[freemine]

Description

with the hope of clarifying building system of TDengine itself, to isolate source code, external project and building-binaries, so that further work, such as test-cases-running, rebuilding among branches, could be simplied and less error prone.

this is still in early stage, and /contrib directory is still containing both source-code and downloaded external projects and there-built-binaries. but it's believed easy to isolate them later on.

wish you guys might think this potentially useful, and looking forward to your comments.

Checklist

Please check the items in the checklist if applicable.

• Is the user manual updated?
• Are the test cases passed and automated?
• Is there no significant decrease in test coverage?

[freemine]

btw, currently, this PR only tested under linux, not including windows or macosx yet.

[freemine]

in order to make external project downloaded to be compiled within the main project of TDengine itself with ADD_SUBDIRECTORY, by setting CMAKE__XXX variable and option(YYY), is really not a good practice, because it ruins the global namespace.
SAY, if project A compiles with CMAKE_XXX set to TRUE, while project B compiles with CMAKE_XXX set FALSE，how ugly the directives in contrib/CMakeLists.txt would be?
with ExternalProject_ADD, you can isolate elegantly easy.

[freemine]

and

which means: no matter you build the TDengine with -DBUILD_CONTRIB:BOOL=ON or not, taosd likely link with /deps//rocksdb_static/librocksdb.a

this really smells bad.

[freemine]

comparison of built binary images