prereq: stop relying on man pages and grep for --recursive on the git-submodule script directly (fixes #21968)

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

SVN-Revision: 48934

include/prereq-build.mk

@@ -155,7 +155,7 @@ $(eval $(call SetupHostCommand,svn,Please install the Subversion client, \
 	svn --version | grep Subversion))
 
 $(eval $(call SetupHostCommand,git,Please install Git (git-core) >= 1.7.12.2, \
-	MANPAGER=cat git submodule --help 2>&1 | grep -q -- --recursive))
+	git --exec-path | xargs -I % -- grep -q -- --recursive %/git-submodule))
 
 $(eval $(call SetupHostCommand,file,Please install the 'file' package, \
 	file --version 2>&1 | grep file))