include: prereq-build.mk: improve gcp check

Adjust the check for gcp (GNU copy command) to rule out false positives
with "Goffi's CoPier" a python copy command.

Fixes FS#218.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

include/prereq-build.mk

@@ -109,8 +109,8 @@ $(eval $(call SetupHostCommand,diff,Please install diffutils, \
 	diff --version 2>&1 | grep diff))
 
 $(eval $(call SetupHostCommand,cp,Please install GNU fileutils, \
-	gcp --help, \
-	cp --help))
+	gcp --help 2>&1 | grep 'Copy SOURCE', \
+	cp --help 2>&1 | grep 'Copy SOURCE'))
 
 $(eval $(call SetupHostCommand,seq,, \
 	gseq --version, \