bpo-44062: Fix PYTHON_FOR_BUILD interpreter lookup for cross-compile builds

Previously, when running configure in a cross-compile scenario, the
PYTHON_FOR_BUILD interpreter could be set to python even if no python
is availble in PATH.

Now, configure will properly set PYTHON_FOR_BUILD based on what is
available in PATH or fail.

Author: vfazio

configure

@@ -2997,12 +2997,11 @@ if test "$cross_compiling" = yes; then
     { $as_echo "$as_me:${as_lineno-$LINENO}: checking for python interpreter for cross build" >&5
 $as_echo_n "checking for python interpreter for cross build... " >&6; }
     if test -z "$PYTHON_FOR_BUILD"; then
-        for interp in python$PACKAGE_VERSION python3 python; do
+        for interp in python$PACKAGE_VERSION python3 python ''; do
 	    which $interp >/dev/null 2>&1 || continue
 	    if $interp -c "import sys;sys.exit(not '.'.join(str(n) for n in sys.version_info[:2]) == '$PACKAGE_VERSION')"; then
 	        break
 	    fi
-            interp=
 	done
         if test x$interp = x; then
 	    as_fn_error $? "python$PACKAGE_VERSION interpreter not found" "$LINENO" 5

configure.ac

@@ -71,12 +71,11 @@ AC_SUBST(PYTHON_FOR_REGEN)
 if test "$cross_compiling" = yes; then
     AC_MSG_CHECKING([for python interpreter for cross build])
     if test -z "$PYTHON_FOR_BUILD"; then
-        for interp in python$PACKAGE_VERSION python3 python; do
+        for interp in python$PACKAGE_VERSION python3 python ''; do
 	    which $interp >/dev/null 2>&1 || continue
 	    if $interp -c "import sys;sys.exit(not '.'.join(str(n) for n in sys.version_info@<:@:2@:>@) == '$PACKAGE_VERSION')"; then
 	        break
 	    fi
-            interp=
 	done
         if test x$interp = x; then
 	    AC_MSG_ERROR([python$PACKAGE_VERSION interpreter not found])