configure: also detect plain grep

We need just 'grep', and we need to be able to call it with 'grep -E'.
Check for 'grep', and bailout if egrep != grep -E

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

Makefile.in

@@ -53,7 +53,7 @@ export mandir          := @mandir@
 export datarootdir     := @datarootdir@
 export install         := @INSTALL@
 export bash            := @_BASH@
-export grep            := @EGREP@
+export grep            := @GREP@
 export make            := @MAKE@
 export sed             := @SED@
 export libtool         := @LIBTOOL@

configure.ac

@@ -91,7 +91,11 @@ AC_ARG_PROGRAM
 # just what I seem to experience...)
 #---------------------------------------------------------------------
 AC_PROG_INSTALL
+AC_PROG_GREP
 AC_PROG_EGREP
+AS_IF(
+    [test ! "$EGREP" = "$GREP -E"],
+    [AC_MSG_ERROR([egrep is not $GREP -E])])
 AC_CACHE_VAL([ac_cv_path_SED],
     [AC_ARG_WITH([sed],
         AS_HELP_STRING([--with-sed=PATH],