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.])
   ],[
   ])
 ])

config/m4/find-func-no-libs2.m4

@@ -42,7 +42,7 @@ case "$ac_res" in
 	yes)
 	eval "ac_cv_func_$1=yes"
 	eval "LIB_$1="
-	AC_DEFINE_UNQUOTED($ac_tr_func)
+	AC_DEFINE_UNQUOTED($ac_tr_func, 1, [Define if the $1 function is available.])
 	AC_MSG_RESULT([yes])
 	;;
 	no)
@@ -53,8 +53,8 @@ case "$ac_res" in
 	*)
 	eval "ac_cv_func_$1=yes"
 	eval "ac_cv_lib_`echo "$ac_res" | sed 's/-l//'`=yes"
-	AC_DEFINE_UNQUOTED($ac_tr_func)
-	AC_DEFINE_UNQUOTED($ac_tr_lib)
+	AC_DEFINE_UNQUOTED($ac_tr_func, 1, [Define if you have the $1 function.])
+	AC_DEFINE_UNQUOTED($ac_tr_lib, 1, [Define if you have the $2 library.])
 	AC_MSG_RESULT([yes, in $ac_res])
 	;;
 esac