configure: make --enable-debug default

This may result in larger binaries by default, and should be harmless
otherwise. Users are advised to use "make install-strip" if they want
binaries without debug symbols.

configure

@@ -386,7 +386,7 @@ Miscellaneous options:
 
 Advanced options:
   --enable-shm              enable shm [autodetect]
-  --enable-debug[=1-3]      compile-in debugging information [disable]
+  --disable-debug           compile-in debugging information [enable]
   --disable-optimization    compile without -O2 [enable]
 
 Use these options if autodetection fails:
@@ -411,7 +411,7 @@ _pkg_config=auto
 _windres=auto
 _cc=auto
 test "$CC" && _cc="$CC"
-_debug=
+_debug=-g
 _opt=-O2
 _cross_compile=no
 _prefix="/usr/local"