Move closer to the standard autotools idiom. (#3187)

* Create `src/config.h` instead of passing everything on the compiler
  command line.

* To reduce the magnitude of this change, add `--include src/config.h`
  to CFLAGS instead of adding `#include "src/config.h"` at the top of
  each source file.  Not all compilers support this, but I don't think
  we care about anything other than gcc and clang at this point.

* Rather than generate `src/config_opts.inc`, emit `JQ_CONFIG` directly
  into `src/config.h`.

* Rather than generate `src/version.h`, define `JQ_VERSION` as an alias
  for the standard `PACKAGE_VERSION`.

scripts/version

@@ -2,7 +2,7 @@
 set -eu
 
 cd "$(dirname "$0")/../"
-test -d .git || exit 1
+test -e .git || exit 1
 
 if git describe --tags --match 'jq-*' >/dev/null 2>&1; then
   git describe --tags --match 'jq-*' --dirty | sed 's/^jq-//'