configure.ac: use AC_PROG_AR

[lanodan]

This way AR is properly queried for including when cross-compiling
where a more graceful fallback than "ar" is needed.

[masatake]

Many checks failed.

[leleliu008]

@lanodan You may want to use AM_PROG_AR rather than AC_PROG_AR, AC_PROG_AR was introduced in autoconf-2.72

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.98%. Comparing base (0498b59) to head (a64287c).
⚠️ Report is 11 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4418   +/-   ##
=======================================
  Coverage   85.98%   85.98%           
=======================================
  Files         255      255           
  Lines       63884    63885    +1     
=======================================
+ Hits        54931    54932    +1     
  Misses       8953     8953           

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[leleliu008]

I think this would be better:

diff --git a/configure.ac b/configure.ac
index 1ed5359e0..29e45f992 100644
--- a/configure.ac
+++ b/configure.ac
@@ -265,9 +265,14 @@ AC_C_INLINE
 #  Typically, this is immediately after AC_PROG_CC, ...
 gl_EARLY
 
+# AC_PROG_AR was introduced in autoconf-2.72
+# AM_PROG_AR was introduced in automake-1.12.0
+# https://lists.gnu.org/archive/html/automake/2012-05/msg00014.html
+m4_ifdef([AC_PROG_AR], [AC_PROG_AR], [m4_ifdef([AM_PROG_AR], [AM_PROG_AR])])
+AC_ARG_VAR([AR], [ar command or path])
 AC_ARG_VAR(RANLIB,[ranlib command or path])
 AC_PROG_RANLIB
-PRETTY_ARG_VAR([AR], [ar command or path], [ar])
+
 AC_C_BIGENDIAN
 
 if test "$cross_compiling" = "yes"; then