Adopt ignore rules for cmake + ninja

In case we have build directory differs from source directory there will be
bunch of files we should ignore, so just remove leading "/" for some or rules.
And fix others.

(cherry picked from commit e2874d95af584b3657eb5dc1d2e5404811dce117)

.gitignore

@@ -84,9 +84,6 @@ cscope*
 /stamp-h1
 /stamp-h2
 
-/include/event2/event-config.h
-/evconfig-private.h
-
 /sample/dns-example
 /sample/event-read-fifo
 /sample/hello-world
@@ -118,29 +115,36 @@ cscope*
 /test/test-changelist
 /test/test-fdleak
 
+**/include/event2/event-config.h
+**/include/evconfig-private.h
 
 # Files generated by cmake
-/CMakeCache.txt
-/CMakeFiles/
-/CTestTestfile.cmake
-/DartConfiguration.tcl
-/LibeventConfig.cmake
-/LibeventConfigVersion.cmake
-/LibeventTargets.cmake
-/bin/
-/cmake_install.cmake
-/include/evconfig-private.h
-/lib/
-/tmp/
-/verify_tests.sh
-/verify_tests.bat
-/event.dir
-/event_core.dir
-/event_extra.dir
+CMakeCache.txt
+CMakeFiles/
+CTestTestfile.cmake
+DartConfiguration.tcl
+LibeventConfig.cmake
+LibeventConfigVersion.cmake
+LibeventTargets.cmake
+bin/
+cmake_install.cmake
+lib/
+tmp/
+verify_tests.sh
+verify_tests.bat
+event.dir
+event_core.dir
+event_extra.dir
 *.vcxproj
 *.sln
 *.filters
 
+# ninja
+build.ninja
+rules.ninja
+.ninja_deps
+.ninja_log
+
 # make dist
 /COPYING
 /INSTALL