[newlib-cygwin] Cygwin: Drop AC_SUBST(build_exeext)

[Jon TURNEY]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=d555072c94c5d705b2f78b5e2d7d4276fc4f5971

commit d555072c94c5d705b2f78b5e2d7d4276fc4f5971
Author: Jon Turney <jon.turney@dronecode.org.uk>
Date:   Wed Oct 21 19:59:26 2020 +0100

    Cygwin: Drop AC_SUBST(build_exeext)
    
    The autoconf variable build_exeext isn't defined, and (since the doc
    subdirectory doesn't build any executables) it's value isn't used.

Diff:
---
 winsup/doc/configure.ac | 2 --
 1 file changed, 2 deletions(-)

diff --git a/winsup/doc/configure.ac b/winsup/doc/configure.ac
index 8364dc79e..32e52415a 100644
--- a/winsup/doc/configure.ac
+++ b/winsup/doc/configure.ac
@@ -20,7 +20,5 @@ AC_CANONICAL_TARGET
 
 AC_PROG_CC
 
-AC_SUBST(build_exeext)
-
 AC_CONFIG_FILES([Makefile])
 AC_OUTPUT