Gentoo enables _FORTIFY_SOURCE by default so this avoids redefined warnings.

--- inkscape-0.91pre3/configure.ac
+++ inkscape-0.91pre3/configure.ac
@@ -72,9 +72,6 @@
   ####
   # Generic cpp flags...
 
-  # What is just plain "-W" ?
-  # Fortify source requires -O2 or higher, which is handled with newer autoconf
-  CPPFLAGS="-W -D_FORTIFY_SOURCE=2 $CPPFLAGS"
   # Enable format and format security warnings
   CPPFLAGS="-Wformat -Wformat-security $CPPFLAGS"
   # Enable all default warnings