[gimp] build/windows: fix Windows build.

[Jehan Pagès <jehanp src gnome org>]

commit 1ed8ceeb1ac6e1416b1cf2a3ac0c17294644f1b3
Author: Jehan <jehan girinstud io>
Date:   Tue Dec 29 18:51:56 2015 +0100

    build/windows: fix Windows build.
    
    `windres` seems a very stupid tool and it breaks with double shlashes
    in parameter paths. Strengthen the rule a little.

 build/windows/gimprc.rule |   10 ++++++----
 1 files changed, 6 insertions(+), 4 deletions(-)
---
diff --git a/build/windows/gimprc.rule b/build/windows/gimprc.rule
index 9e35bf5..5e3b10d 100644
--- a/build/windows/gimprc.rule
+++ b/build/windows/gimprc.rule
@@ -2,10 +2,12 @@
 
 GIMPAPPRC = $(top_builddir)/build/windows/gimp.rc
 
-%.rc.o: 
+# `windres` seems a very stupid tool and it breaks with double shlashes
+# in parameter paths. Strengthen the rule a little.
+%.rc.o:
        $(WINDRES) --define ORIGINALFILENAME_STR="$*$(EXEEXT)" \
                --define INTERNALNAME_STR="$*" \
-               --define TOP_SRCDIR="$(top_srcdir)" \
-               -I$(top_srcdir)/app \
-               -I$(top_builddir)/app \
+               --define TOP_SRCDIR="`echo $(top_srcdir) | sed 's*//*/*'`" \
+               -I"`echo $(top_srcdir)/app | sed 's%/\+%/%'`" \
+               -I"`echo $(top_builddir)/app | sed 's%/\+%/%'`"\
                $(GIMPAPPRC) $@