Cmake sometimes generates garbage in 'unittest/gunit/CMakeFiles/merge_large_tests-t.dir/build.make' after downloading 'googletest-release-1.8.0.zip'

When '-DENABLE_DOWNLOADS=1' option is specified and there is no local version of 'source_downloads/googletest-release-1.8.0.zip', sometimes cmake generates corrupted 'unittest/gunit/CMakeFiles/merge_large_tests-t.dir/build.make' makefile which includes SSL handshake garbage.

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# External object files for target merge_large_tests-t
merge_large_tests__t_EXTERNAL_OBJECTS =

unittest/gunit/SSLv3,\ TLS\ handshake,\ Client\ hello\ (1):

merge_large_tests-t: unittest/gunit/CMakeFiles/merge_large_tests-t.dir/merge_large_tests.cc.o
unittest/gunit/SSLv3,\ TLS\ handshake,\ Client\ hello\ (1):

merge_large_tests-t: unittest/gunit/CMakeFiles/merge_large_tests-t.dir/__/__/storage/example/ha_example.cc.o
unittest/gunit/SSLv3,\ TLS\ handshake,\ Client\ hello\ (1):

merge_large_tests-t: unittest/gunit/CMakeFiles/merge_large_tests-t.dir/build.make
unittest/gunit/SSLv3,\ TLS\ handshake,\ Client\ hello\ (1):

merge_large_tests-t: sql/libsql.a
*********************************

This causes build failures.

Observed on 5.6.35 on centos5-64, debian-wheezy-x64 and ubuntu-xenial-64bit.