RFR: 8351029: IncludeCustomExtension does not work on cygwin with source code below /home [v2]

[Magnus Ihse Bursie]

> If you check out your source code in Cygwin, somewhere under /home/... (as opposed to /cygpath/...), IncludeCustomExtension does not work.
> 
> The problem is that TOPDIR gets a different lexical value from the spec.gmk file when setup in Makefile, even if this was the same directory. Since THIS_INCLUDE is calculated as a relative path by string substitution of $(TOPDIR), the path failed to relativize correctly.

Magnus Ihse Bursie has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Merge branch 'master' into fix-preinit-topdir
 - Alternative solution
 - 8351029: IncludeCustomExtension does not work on cygwin with source code below /home

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/25100/files
  - new: https://git.openjdk.org/jdk/pull/25100/files/65a1b222..f227d802

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25100&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25100&range=00-01

  Stats: 1586 lines in 89 files changed: 925 ins; 309 del; 352 mod
  Patch: https://git.openjdk.org/jdk/pull/25100.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25100/head:pull/25100

PR: https://git.openjdk.org/jdk/pull/25100