[LyX/master] Add explicit check for EGREP in configure.ac

[Jean-Marc Lasgouttes]

commit 31cde7247638a50c2302744984732064505b7d85
Author: Jean-Marc Lasgouttes <lasgouttes at lyx.org>
Date:   Fri Jul 12 16:02:53 2024 +0200

    Add explicit check for EGREP in configure.ac
    
    It is used in lib/Makefile.am and we were relying some autoconf macros
    to define it as a side-effect. This is not the case anymore as of
    autoconf 2.72.
---
 configure.ac | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/configure.ac b/configure.ac
index 4be32fa9c0..c796b60f9d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -122,6 +122,9 @@ AC_SUBST([HAVE_WCHAR_T])
 # Needed for Mingw-w64
 AC_TYPE_LONG_LONG_INT
 
+# egrep is used in lib/Makefile.am
+AC_PROG_EGREP
+
 ### We need iconv for unicode support (Qt frontend requires it too)
 LYX_USE_INCLUDED_ICONV