Fixing test build on gcc

appveyor.yml

@@ -21,6 +21,7 @@ environment:
     GENERATOR: "Visual Studio 15 2017"
     GROUP: "SUPPORTED"
     BOOST_ROOT: "C:/Libraries/boost_1_67_0"
+
     PLATFORM: "win32"
     REF_BUILD_ID: "220dev5"
   # New build on Visual Studio 15 2017
@@ -37,6 +38,7 @@ init:
   - set EPANET_HOME=%APPVEYOR_BUILD_FOLDER%
   - set BUILD_HOME=buildprod
   - set TEST_HOME=nrtestsuite
+  - set PATH=%PATH%;%BOOST_ROOT%/
 
   # See values set
   - echo %APPVEYOR_BUILD_WORKER_IMAGE%