configure: Move target source selection into configure.host

This eliminates the AM_CONDITIONAL ugliness, which eliminates
just a bit of extra boilerplate for a new target.

At the same time, properly categorize the EXTRA_DIST files
into SOURCES and HEADERS, for the generation of ctags.
testsuite: Detect clang

Clang doesn't like the -Wno-psabi argument that we want to pass to GCC.
Since clang is detected as GCC via __GNUC__, use ax_cv_c_compiler_vendor.
testsuite: Use feature test rather than enumeration for complex
testsuite: Move complex tests to their own subdirectory

It seems a bit silly to isolate them by globbing vs "*complex*"
when we can just as easily put them in their own subdirectory.
testsuite: Add trivial tests for Go closures
x86: Fix typo in ffi_prep_go_closure

Used the wrong register for THISCALL and FASTCALL.