configure: Add --extra-filename flag

This mixes in additional information into the hash that is
passed to -C extra-filename. It can be used to further distinguish
the standard libraries if they must be installed next to each
other.

Closes #29559

configure

@@ -622,6 +622,7 @@ valopt aarch64-linux-android-ndk "" "aarch64-linux-android NDK standalone path"
 valopt nacl-cross-path  "" "NaCl SDK path (Pepper Canary is recommended). Must be absolute!"
 valopt release-channel "dev" "the name of the release channel to build"
 valopt musl-root "/usr/local" "MUSL root installation directory"
+valopt extra-filename "" "Additional data that is hashed and passed to the -C extra-filename flag"
 
 # Used on systems where "cc" and "ar" are unavailable
 valopt default-linker "cc" "the default linker"

mk/main.mk

@@ -22,7 +22,7 @@ CFG_PRERELEASE_VERSION=.1
 
 # Append a version-dependent hash to each library, so we can install different
 # versions in the same place
-CFG_FILENAME_EXTRA=$(shell printf '%s' $(CFG_RELEASE) | $(CFG_HASH_COMMAND))
+CFG_FILENAME_EXTRA=$(shell printf '%s' $(CFG_RELEASE)$(CFG_EXTRA_FILENAME) | $(CFG_HASH_COMMAND))
 
 ifeq ($(CFG_RELEASE_CHANNEL),stable)
 # This is the normal semver version string, e.g. "0.12.0", "0.12.0-nightly"