Don't force the use of DEBUG on application in autoconf macros

If WX_STANDARD_OPTIONS() is not used, we shouldn't add any debugging nor
optimization options to C{,XX}FLAGS as the application presumably handles it
on its own, it was completely unexpected to see -O2 appear in CXXFLAGS.

Fix this by only doing anything if DEBUG variable is defined. Even this is not
optimal as it's such a common name that it could clash with an unrelated
variable in the application configure script, but doing anything else risks
breaking existing scripts relying on it being named like this.

wxwin.m4

@@ -928,11 +928,8 @@ AC_DEFUN([WX_DETECT_STANDARD_OPTION_VALUES],
                          ])
         fi
 
-        dnl now we can finally update the DEBUG,UNICODE,SHARED options
-        dnl to their final values if they were not already set
-        if test -z "$DEBUG" ; then
-            DEBUG=$WX_DEBUG
-        fi
+        dnl now we can finally update the options to their final values if they
+        dnl were not already set
         if test -z "$UNICODE" ; then
             UNICODE=$WX_UNICODE
         fi
@@ -943,20 +940,17 @@ AC_DEFUN([WX_DETECT_STANDARD_OPTION_VALUES],
             TOOLKIT=$WX_PORT
         fi
 
-        dnl in case the user needs a BUILD=debug/release var...
-        if test "$DEBUG" = "1"; then
-            BUILD="debug"
-        elif test "$DEBUG" = "0" -o "$DEBUG" = ""; then
-            BUILD="release"
-        fi
-
-        dnl respect the DEBUG variable adding the optimize/debug flags
+        dnl respect the DEBUG variable adding the optimize/debug flags and also
+        dnl define a BUILD variable in case the user wants to use it
+        dnl
         dnl NOTE: the CXXFLAGS are merged together with the CPPFLAGS so we
         dnl       don't need to set them, too
         if test "$DEBUG" = "1"; then
+            BUILD="debug"
             CXXFLAGS="$CXXFLAGS -g -O0"
             CFLAGS="$CFLAGS -g -O0"
-        else
+        elif test "$DEBUG" = "0"; then
+            BUILD="release"
             CXXFLAGS="$CXXFLAGS -O2"
             CFLAGS="$CFLAGS -O2"
         fi