Use $(filter ...) instead of $(findstring ...)

To properly extract space separated options from DEB_BUILD_OPTIONS.

debian/changelog

@@ -26,6 +26,8 @@ dpkg (1.15.0) UNRELEASED; urgency=low
   * Properly lstat the correct file when using --root on package upgrade.
     Thanks to Egmont Koblinger. Closes: #281057
   * Print a longer string when a disallowed field value is found when parsing.
+  * Use $(filter ...) instead of $(findstring ...) to extract space separated
+    options from DEB_BUILD_OPTIONS in debian/rules.
 
   [ Raphael Hertzog ]
   * Enhance dpkg-shlibdeps's error message when a library can't be found to

debian/rules

@@ -8,7 +8,7 @@ CFLAGS = -g $(WFLAGS)
 CXXFLAGS = -g $(WFLAGS)
 
 # Disable optimisations if ‘noopt’ found in $DEB_BUILD_OPTIONS
-ifneq (,$(findstring noopt,$(DEB_BUILD_OPTIONS)))
+ifneq (,$(filter noopt,$(DEB_BUILD_OPTIONS)))
 	CFLAGS += -O0
 	CXXFLAGS += -O0
 else
@@ -66,7 +66,7 @@ build: build-tree/config.status
 check: build
 	dh_testdir
 
-ifeq (,$(findstring nocheck,$(DEB_BUILD_OPTIONS)))
+ifeq (,$(filter nocheck,$(DEB_BUILD_OPTIONS)))
 	cd build-tree && $(MAKE) check
 endif