build: fix C string encoding for `PRODUCT_DIR_ABS`

[addaleax]

Since the PRODUCT_DIR_ABS gyp variable is meant to be used in a C string in the OpenSSL config, provide a version of it that actually provides it in a way that is always usable as a C string. Otherwise, unescaped characters in the path can mess with the string definitions; for example, building in paths on Windows whose directories start with valid or invalid escape sequences (e.g.: C:\...\x61foobar\... or C:\...\456789\...) can result in failing builds or incorrect paths provided to OpenSSL.

Needs: https://github.com/nodejs/gyp-next/pull/271