RFR: Improve and modernize the makefile

[Magnus Ihse Bursie]

A bunch of fixes to the make file, to use more modern/robust syntax, and to keep intermediate build results.

This is a prerequisite for any further changes to the make file.

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Commit messages:
 - Improve and modernize the makefile

Changes: https://git.openjdk.java.net/guide/pull/46/files
 Webrev: https://webrevs.openjdk.java.net/?repo=guide&pr=46&range=00
  Stats: 32 lines in 1 file changed: 12 ins; 10 del; 10 mod
  Patch: https://git.openjdk.java.net/guide/pull/46.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/46/head:pull/46

PR: https://git.openjdk.java.net/guide/pull/46