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.

CMakeLists.txt

@@ -86,6 +86,8 @@ foreach(install_dir ${CMAKE_INSTALL_FULL_BINDIR}
     break()
   endif()
 endforeach()
+message(STATUS "CMAKE_INSTALL_FULL_DATADIR: ${CMAKE_INSTALL_FULL_DATADIR}")
+message(STATUS "DESTDIR: ${DESTDIR}")
 
 # GnuCash installs two files in ${CMAKE_INSTALL_SYSCONFDIR}
 set(BINDIR ${CMAKE_INSTALL_BINDIR} CACHE STRING "user executables")

gnucash/gschemas/CMakeLists.txt

@@ -32,10 +32,20 @@ if (COMPILE_GSCHEMAS)
 
     add_custom_target(compiled-schemas ALL DEPENDS ${SCHEMADIR_BUILD}/gschemas.compiled)
 
-
-    install(CODE "execute_process(
-        COMMAND ${SHELL} -c \"echo Compiling gschema files in $DESTDIR${CMAKE_INSTALL_FULL_DATADIR}/glib-2.0/schemas ;
-                              ${GLIB_COMPILE_SCHEMAS} $DESTDIR${CMAKE_INSTALL_FULL_DATADIR}/glib-2.0/schemas\")")
+    # On Windows concatenating two absolute paths results in an invalid path (having two drive letters)
+    # If DESTDIR is not set on the other hand, the below install command works just fine
+    # So verify DESTDIR is not set on Windows
+    # Note we have to do this at build time, not configure time so the guard is part of the custom install command
+    install(CODE "
+    if (WIN32)
+        set (DESTDIR \$ENV\{DESTDIR\})
+        if (DESTDIR)
+            message(SEND_ERROR \"GnuCash can't be built with the DESTDIR environment variable set on Windows (due to bad interference with glib-compile-schemas).\")
+        endif()
+    endif()
+    execute_process(
+        COMMAND ${SHELL} -c \"echo Compiling gschema files in \$ENV\{DESTDIR\}${CMAKE_INSTALL_FULL_DATADIR}/glib-2.0/schemas ;
+                              ${GLIB_COMPILE_SCHEMAS} \$ENV\{DESTDIR\}${CMAKE_INSTALL_FULL_DATADIR}/glib-2.0/schemas\")")
 endif ()
 
 set(gschemas_DIST_local "")