[C++] Builds with ARROW_JEMALLOC=ON wait until jemalloc_ep is complete before building any libarrow .cc files

[asfimport]

This has been a frustration for me that my builds wait for the jemalloc_ep build to complete before proceeding to compile any C++ files for libarrow. Since only memory_pool.cc depends on jemalloc_ep, there must be some way to create a dependency between that single C++ file and jemalloc_ep. I spent a few minutes Googling about it but did not come up with an easy answer

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

PRs and other links:

• GitHub Pull Request #5281

_{Note: This issue was originally created as ARROW-6447. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
+1. Do you know what the culprit is? Perhaps add_include_directories?

[asfimport]

Wes McKinney / @wesm:
All of the toolchain libraries are dependencies of the entire arrow_objlib target, so the build graph blocks until jemalloc_ep is complete. Really jemalloc_ep should only be a dependency of compiling memory_pool.cc but I haven't figured out how to convince CMake to do that

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 5281
#5281