Bug 1644887 - `configure` calls into `$PYTHON3` rather than `python3` directly r=glandium

Especially after bug 1622963 landed, the system `python3` executable as reported by `which` may not be compatible with the build system (because it might be Python 3.5, for one). You can configure a custom `python3` executable by setting the `PYTHON3` environment variable, but `configure` just calls directly into `python3` regardless of whatever you might have configured `$PYTHON3` to be, so we fix that here.

Also remove an explanatory comment that may have made sense 5 years ago but is no longer relevant.

Differential Revision: https://phabricator.services.mozilla.com/D79187

configure.in

@@ -3,12 +3,6 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 #
-# Welcome to the new world of configure. We're working on moving to a
-# python based configure. In the meanwhile, remains of the old autoconf
-# based configure is in old-configure.in.
-# If you need to add something, please come talk to the build system
-# peers for now.
-#
 # Because adding a configure file in the tree is going to conflict with
 # existing configure files in people's (and automation) work trees, and
 # because some automation jobs are still running autoconf and configure
@@ -20,6 +14,7 @@
 
 SRCDIR=$(dirname $0)
 TOPSRCDIR="$SRCDIR"
+PYTHON3="${PYTHON3:-python3}"
 export OLD_CONFIGURE="$SRCDIR"/old-configure
 
-exec python3 "$TOPSRCDIR/configure.py" "$@"
+exec "$PYTHON3" "$TOPSRCDIR/configure.py" "$@"

js/src/configure.in

@@ -3,12 +3,6 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 #
-# Welcome to the new world of configure. We're working on moving to a
-# python based configure. In the meanwhile, remains of the old autoconf
-# based configure is in old-configure.in.
-# If you need to add something, please come talk to the build system
-# peers for now.
-#
 # Because adding a configure file in the tree is going to conflict with
 # existing configure files in people's (and automation) work trees, and
 # because some automation jobs are still running autoconf and configure
@@ -20,8 +14,9 @@
 
 SRCDIR=$(dirname $0)
 TOPSRCDIR="$SRCDIR"/../..
+PYTHON3="${PYTHON3:-python3}"
 export OLD_CONFIGURE="$SRCDIR"/old-configure
 
 set -- "$@" --enable-project=js
 
-exec python3 "$TOPSRCDIR/configure.py" "$@"
+exec "$PYTHON3" "$TOPSRCDIR/configure.py" "$@"