dpkg-buildpackage: Ignore DEB_CHECK_COMMAND if the command is not found

This is an environment setting, that might percolate to chroots and
similar, so it is not wise to fail hard in case the command is not
found, contrary to when it is passed on the command line. Just ignore
the setting in that case.

debian/changelog

@@ -87,6 +87,7 @@ dpkg (1.17.11) UNRELEASED; urgency=low
     Thanks to Sylvestre Ledru <sylvestre@debian.org>. Closes: #756230
   * Delete the current compressor SIGPIPE disposition in Dpkg::Source::Archive,
     which fixes ignoring SIGPIPE from the calling process. Closes: #756526
+  * Ignore DEB_CHECK_COMMAND in dpkg-buildpackage if the command is not found.
 
   [ Updated programs translations ]
   * Danish (Joe Dalton). Closes: #754127

scripts/dpkg-buildpackage.pl

@@ -188,6 +188,8 @@ my $build_opts = Dpkg::BuildOptions->new();
 
 if ($build_opts->has('nocheck')) {
     $check_command = undef;
+} elsif (not find_command($check_command)) {
+    $check_command = undef;
 }
 
 while (@ARGV) {