update docs to list cmake required for build from source page

[azai91]

Description

https://mxnet.incubator.apache.org/install/build_from_source.html

currently our docs state that cmake is recommended but not required. in the process to migrate fully to cmake (and because it is required by some of our builds already) we will state that cmake is a prerequisite for building from source.

there were already two public disussions to remove cmake (one vote and one on github) so this transition is happening:
https://lists.apache.org/list.html?dev@mxnet.apache.org:2018-6:cmake
#8702

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [MXNET-$JIRA_ID], where $JIRA_ID refers to the relevant JIRA issue created (except PRs with tiny changes)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage:
• Unit tests are added for small changes to verify correctness (e.g. adding a new operator)
• Nightly tests are added for complicated/long-running ones (e.g. changing distributed kvstore)
• Build tests will be added for build configuration changes (e.g. adding a new build option with NCCL)
• Code is well-documented:
• For user-facing API changes, API doc string has been updated.
• For new C++ functions in header files, their functionalities and arguments are documented.
• For new examples, README.md is added to explain the what the example does, the source of the dataset, expected performance on test set and reference to the original paper if applicable
• Check the API doc at http://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-$PR_ID/$BUILD_ID/index.html
• To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

Changes

• move cmake to required section in build from source page.

Comments

• If this change is a backward incompatible change, why must this change be made.
• Interesting edge cases to note here

[zhreshold]

can you elaborate why cmake is required? I am building from source just fine with config.mk and make

[azai91]

when building with make with the MKLDNN, cmake it required: https://github.com/apache/incubator-mxnet/blob/master/mkldnn.mk#L40.

additionally there two discussions already to remove the makefile and make cmake default

https://lists.apache.org/list.html?dev@mxnet.apache.org:2018-6:cmake
#8702

[zhreshold]

for GNU make, we can continue stay with MKLDNN=0 regardless of final discussion result. Anyways, I think it should help some users to skip those compiling tools in certain circumstances.

[azai91]

the purpose of the vote was to eliminate the Makefile. if you think we should keep the Makefile, raise it in the email list. otherwise the direction we are heading is removing it completely and becoming dependent on cmake.

[stu1130]

Thanks for your contributions @azai91
@mxnet-label-bot[pr-awaiting-response]

[azai91]

@zhreshold updated the docs to specify that cmake is only necessary for MKLDNN.

[lupesko]

Looks good.