Unable to build UE4 from source

As the title says, I am getting a failure when I try to build the engine from source. I have followed the readme on github, and I’ve even been able to compile successfully in the past (a month ago, or so). Now when I try, I get the following error:

1>------ Build started: Project: UE4, Configuration: Development_Editor x64 ------
1>  Building UnrealHeaderTool...
1>  Performing 1 actions (4 in parallel)
1>  [1/1] Link UnrealHeaderTool.exe
1>  -------- End Detailed Actions Stats -----------------------------------------------------------
1>  Cumulative action seconds (8 processors): 0.00 building projects, 0.00 compiling, 0.00 creating app bundles, 0.00 generating debug info, 0.53 linking, 0.00 other
1>  UBT execution time: 4.41 seconds
1>  Parsing headers for UE4Editor
1>EXEC : error : System.ComponentModel.Win32Exception (0x80004005): The system cannot find the file specified
1>     at System.Diagnostics.Process.StartWithCreateProcess(ProcessStartInfo startInfo)
1>     at System.Diagnostics.Process.Start(ProcessStartInfo startInfo)
1>     at UnrealBuildTool.ExternalExecution.RunExternalExecutable(String ExePath, String Commandline) in c:\Users\Bradley\Desktop\UnrealEngine\Engine\Source\Programs\UnrealBuildTool\System\ExternalExecution.cs:line 518
1>     at UnrealBuildTool.ExternalExecution.ExecuteHeaderToolIfNecessary(UEBuildTarget Target, CPPEnvironment GlobalCompileEnvironment, List`1 UObjectModules, String ModuleInfoFileName, ECompilationResult& UHTResult) in c:\Users\Bradley\Desktop\UnrealEngine\Engine\Source\Programs\UnrealBuildTool\System\ExternalExecution.cs:line 653
1>     at UnrealBuildTool.UEBuildTarget.Build(IUEToolChain TargetToolChain, List`1& OutputItems, List`1& UObjectModules, String& EULAViolationWarning) in c:\Users\Bradley\Desktop\UnrealEngine\Engine\Source\Programs\UnrealBuildTool\Configuration\UEBuildTarget.cs:line 1541
1>     at UnrealBuildTool.UnrealBuildTool.RunUBT(String[] Arguments) in c:\Users\Bradley\Desktop\UnrealEngine\Engine\Source\Programs\UnrealBuildTool\System\UnrealBuildTool.cs:line 1590
1>C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\V120\Microsoft.MakeFile.Targets(38,5): error MSB3073: The command "..\..\Build\BatchFiles\Build.bat UE4Editor Win64 Development" exited with code -1.
========== Build: 0 succeeded, 1 failed, 1 up-to-date, 0 skipped ==========

Solved. It turns out it was Avast antivirus which was silently blocking the UnrealBuildTool from working. This makes this a duplicate of Avast! Free Antivirus false positive UnrealBuildTool.exe - AnswerHub - Unreal Engine Forums