[xdt-features.m4] always enable whole set of compilation warnings

Currently, compilation warnings are enabled only for --enable-debug=yes and --enable-debug=full. It would be nice to always enable them, especially for --disable-debug where some parts of the code may be disabled by the fact that NDEBUG is set.

Also, -Wshadow is enabled only for --enable-debug=yes, not for --enable-debug=full. What is the reason for this?

In short, I would be for enabling this set of compilation warnings all the time:

  -DXFCE_DISABLE_DEPRECATED \
  -Wall -Wextra \
  -Wno-missing-field-initializers \
  -Wno-unused-parameter -Wold-style-definition \
  -Wdeclaration-after-statement \
  -Wmissing-declarations \
  -Wmissing-noreturn -Wpointer-arith \
  -Wcast-align -Wformat -Wformat-security -Wformat-y2k \
  -Winit-self -Wmissing-include-dirs -Wundef \
  -Wnested-externs -Wredundant-decls -Wshadow \

NB: except for -Wredundant-decls that should apparently be disabled on OpenBSD.