gh-145961: Fix typos in configure.ac and pyconfig.h.in

- configure.ac line 6415: "useable" -> "usable" (two occurrences)
- configure.ac line 6505: "defauly" -> "default"
- pyconfig.h.in line 1602: "useable" -> "usable" (two occurrences)

All changes are in comments only.

Author: crawfordxx

configure.ac

@@ -6412,7 +6412,7 @@ if test "$ac_cv_sizeof_wchar_t" -ge 2 \
           -a "$ac_cv_wchar_t_signed" = "no"
 then
   AC_DEFINE([HAVE_USABLE_WCHAR_T], [1],
-  [Define if you have a useable wchar_t type defined in wchar.h; useable
+  [Define if you have a usable wchar_t type defined in wchar.h; usable
    means wchar_t must be an unsigned type with at least 16 bits. (see
    Include/unicodeobject.h).])
   AC_MSG_RESULT([yes])
@@ -6502,7 +6502,7 @@ fi
 # Check for --with-platlibdir
 # /usr/$PLATLIBDIR/python$(VERSION)$(ABI_THREAD)
 AC_SUBST([PLATLIBDIR])
-PLATLIBDIR="lib"  # XXX: We should probably calculate the defauly from libdir, if defined.
+PLATLIBDIR="lib"  # XXX: We should probably calculate the default from libdir, if defined.
 AC_MSG_CHECKING([for --with-platlibdir])
 AC_ARG_WITH(
   [platlibdir],

pyconfig.h.in

@@ -1599,7 +1599,7 @@
 /* Define to 1 if you have the 'unshare' function. */
 #undef HAVE_UNSHARE
 
-/* Define if you have a useable wchar_t type defined in wchar.h; useable means
+/* Define if you have a usable wchar_t type defined in wchar.h; usable means
    wchar_t must be an unsigned type with at least 16 bits. (see
    Include/unicodeobject.h). */
 #undef HAVE_USABLE_WCHAR_T