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`.

config/m4/check-math-func.m4

@@ -2,8 +2,8 @@ dnl AC_CHECK_MATH_FUNC(func)
 AC_DEFUN([AC_CHECK_MATH_FUNC], [
   AC_LANG(C)
   AC_CHECK_LIB([m],[$1],[
-    eval "ac_tr_func=HAVE_[]upcase($1)"
-    AC_DEFINE_UNQUOTED($ac_tr_func)
+    m4_define([ac_tr_func], [HAVE_]m4_toupper($1))
+    AC_DEFINE(ac_tr_func, 1, [Define if you have the $1 math function.])
   ],[
   ])
 ])