gnucash maint: Tweak install rule to be able to compile glib's schema's on Windows as well

[Geert Janssens]

Updated	 via  https://github.com/Gnucash/gnucash/commit/b00a95c0 (commit)
	from  https://github.com/Gnucash/gnucash/commit/badda882 (commit)



commit b00a95c0b39e2666eac09bf13304782492d886b8
Author: Geert Janssens <geert at kobaltwit.be>
Date:   Sun Oct 6 21:27:10 2019 +0200

    Tweak install rule to be able to compile glib's schema's on Windows as well
    
    (Cherry picked from master)
    It's a bit of a hack based on the assumption DESTDIR is never set on Windows.
    A install time guard is added to assert this.
    
    It needed a few changes to make this working:
    - Have cmake expand DESTDIR instead of delaying this to bash
      If not, bash would see "$DESTDIRC:/gcdev64/..." and we'd loose
      the drive letter in bash' expansion of $DESTDIRC.
      So work with $ENV{DESTDIR} instead
    - To prevent cmake from already expanding this in the
      build system generation step add the appropriate escapes to
      that variable.
    - Add guard code in the install command that asserts
      DESTDIR is not set on Windows. Use similar escapes as
      necessary to ensure the evaluation happens at install time
      rather than in the generation step.



Summary of changes:
 CMakeLists.txt                  |  2 ++
 gnucash/gschemas/CMakeLists.txt | 18 ++++++++++++++----
 2 files changed, 16 insertions(+), 4 deletions(-)