cmake: Ignore build subdirectories within source directory

Author: hebasto

.gitignore

@@ -1,15 +1,10 @@
-bench_ecmult
-bench_internal
-noverify_tests
-tests
-exhaustive_tests
-precompute_ecmult_gen
-precompute_ecmult
-ctime_tests
-ecdh_example
-ecdsa_example
-schnorr_example
-ellswift_example
+# Build subdirectories.
+/*build*
+!/build-aux
+!/build_msvc
+
+*.tar.gz
+
 *.exe
 *.so
 *.a