Verify .gitignore entries

This change adds a make target 'vcscheck' which scans the git workspace
for new, untracked files missing from the .gitignore configuration; this
is done to help prevent adding unwanted build artifacts to the source
tree during development.

Reviewed-by: Neal Gompa <ngompa@datto.com>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #8281

.gitignore

@@ -60,3 +60,4 @@ cscope.*
 *.tar.gz
 *.patch
 *.orig
+*.log

Makefile.am

@@ -75,7 +75,7 @@ install-data-hook:
 	ln -fs zfs.release spl.release
 endif
 
-codecheck: cstyle shellcheck flake8 mancheck testscheck
+codecheck: cstyle shellcheck flake8 mancheck testscheck vcscheck
 
 checkstyle: codecheck commitcheck
 
@@ -117,6 +117,12 @@ testscheck:
 		xargs -r stat -c '%A %n' | \
 		awk '{c++; print} END {if(c>0) exit 1}'
 
+vcscheck:
+	@if git rev-parse --git-dir > /dev/null 2>&1; then \
+		git ls-files . --exclude-standard --others | \
+		awk '{c++; print} END {if(c>0) exit 1}' ; \
+	fi
+
 lint: cppcheck paxcheck
 
 cppcheck: