Fix configure check for Boehm GC library.

The configure script could fail on Gentoo Linux as there was not always
a static ``libgc.a`` available. We now check for the file that we need
on each platform.

This was reported by Patrick Lauer.
Merge pull request #377 from waywardmonkeys/autoconf-gc-fix

Fix configure check for Boehm GC library.
Bug 7440: Add support for different thread-local models in LLVM 3.2

* sources/lib/llvm/llvm-global.dylan
  (<llvm-global-thread-local-kind>): New constant enumeration.
  (llvm-global-variable-thread-local): Change type and rename from
   llvm-global-variable-thread-local?.

* sources/lib/llvm/llvm-bitcode.dylan
  (write-module): Support the other TLS model encodings
  (thread-local-encoding): New function for encoding TLS models.

* sources/lib/llvm/llvm-asm-parser.dylgram
  (definition rules): Change to match changed <llvm-global> init keyword.
  (opt-thread-local rules): Add tls-model case.
  (tls-model rules): New nonterminal.

* sources/lib/llvm/llvm-asm-reader.dylan
  ($llvm-keywords): Add new TLS model keywords.
Bug 7458: Change LLVM type conventions for primitive-vector-element

* sources/dfmc/llvm-back-end/llvm-ops.dylan
  (op--getslotptr): Ensure that an LLVM type is computed for the result.

* sources/dfmc/llvm-back-end/llvm-primitives-basic.dylan
  (primitive-vector-element): Change to mapped-parameter.
  (primitive-vector-element-setter): Change to not assume mapped-parameter.
Bug 7458: Implement primitive-apply

* sources/dfmc/llvm-back-end/llvm-primitives-lambda.dylan
  ($maximum-argument-count): New constant
  (primitive-apply): Implement to retrieve argument values from a
   <simple-object-vector> and pass them the the given function's XEP.
Bug 7458: Improve LLVM code generation Makefile test jigs

* Makefile.in (cli3, rtg, bootstrap-stage-4, cli4, testc, testl, xtestl):
  Update to adapt to Makefile changes.
Add strings library documentation.
Addressed brucem's comments on strings pull request.
strings doc: Fix split-lines example.
strings doc: Lowercase the library name in the TOC, to match the rest.
Merge pull request #379 from cgay/strings-doc

Add strings library documentation.
strings doc: one more capitalization fix