[source] include: prereq-build.mk: improve gcp check

[LEDE Commits]

jow pushed a commit to source.git, branch master:
https://git.lede-project.org/b81fc29123f777f61d99c59c214a69e253cb2cc0

commit b81fc29123f777f61d99c59c214a69e253cb2cc0
Author: Jo-Philipp Wich <jo at mein.io>
AuthorDate: Tue Oct 11 09:33:18 2016 +0200

    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 at mein.io>
---
 include/prereq-build.mk | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/prereq-build.mk b/include/prereq-build.mk
index 4a0509a..ed652ca 100644
--- a/include/prereq-build.mk
+++ b/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, \