build(jemalloc): Install Jemalloc if missing via Makefile.

[jarifibrahim]

This PR adds a jemalloc target to makefile to install jemalloc. The make install command will install jemalloc if it's not already installed.

The make jemalloc target currently fails on docker with command sudo not found. I don't have a solution for it.

The change doesn't work on macOS or Windows. Dgraph builds with jemalloc by default on Linux only.

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This change is 

[danielmai]

Reviewable status: 0 of 1 files reviewed, 1 unresolved discussion (waiting on @jarifibrahim, @manishrjain, and @vvbalaji-dgraph)

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dgraph/Makefile, line 98 at r1 (raw file):

ldconfig ; \

Does this need sudo too? I get this error locally, and it happens in CI too.

$ ldconfig                   
/sbin/ldconfig.real: Can't create temporary cache file /etc/ld.so.cache~: Permission denied

Do we need to run ldconfig? Isn't sudo make install sufficient?

[danielmai]

Reviewable status: 0 of 1 files reviewed, 2 unresolved discussions (waiting on @jarifibrahim, @manishrjain, and @vvbalaji-dgraph)

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dgraph/Makefile, line 66 at r1 (raw file):

ldconfig -p

We're using the static library (libjemalloc.a), not the shared library (libjemalloc.so), so we shouldn't need to use ldconfig, as it doesn't tell us about the .a file:

$ ldconfig -p | grep jemalloc
	libjemalloc.so.2 (libc6,x86-64) => /usr/local/lib/libjemalloc.so.2
	libjemalloc.so (libc6,x86-64) => /usr/local/lib/libjemalloc.so

We should check if the path to the archive exists: /usr/local/lib/libjemalloc.a