Added --force-version=STRING option to configure.

Details:
- Added an option to configure that allows the user to force an arbitrary
  version string at configure-time. The help text also now describes the
  usage information.
- Changed the way the version string is communicated to the Makefile.
  Previously, it was read into the VERSION variable from the 'version' file
  via $(shell cat ...). Now, the VERSION variable is instead set in
  config.mk (via a configure-substituted anchor from config.mk.in).

Makefile

@@ -85,9 +85,6 @@ TESTSUITE_CONF_GEN := input.general
 TESTSUITE_CONF_OPS := input.operations
 TESTSUITE_OUT_FILE := output.testsuite
 
-# The name of the file where the version string is stored.
-VERSION_FILE       := version
-
 # The name of the "special" directories, which contain source code that
 # use non-standard compiler flags.
 NOOPT_DIR          := noopt
@@ -141,7 +138,6 @@ BASE_LIB_PATH           := ./$(LIB_DIR)/$(CONFIG_NAME)
 
 # Construct the architecture-version string, which will be used to name the
 # library upon installation.
-VERSION                := $(shell cat $(DIST_PATH)/$(VERSION_FILE))
 VERS_CONF              := $(VERSION)-$(CONFIG_NAME)
 
 # --- Library names ---