title: Qt creator: include path for external lib not found
body: <p>I'm trying to add net-snmp lib to my project using win7 + cygwin:
  <img src="https://i.stack.imgur.com/1iBTT.png" alt="Add library"></p>
  
  <p>Project file seems to contain valid lib and path entries:</p>
  
  <blockquote>
    <p>win32:CONFIG(release, debug|release): LIBS += -L$$PWD/C:/usr/lib/
    -lnetsnmp else:win32:CONFIG(debug, debug|release): LIBS += -L$$PWD/C:/usr/lib/ -lnetsnmpd</p>
    
    <p>INCLUDEPATH += $$PWD/C:/usr/include DEPENDPATH += $$PWD/C:/usr/include</p>
  </blockquote>
  
  <p>But I can't build the project, because include file is not found, also it exists under given directory physically (c:/usr/include/net-snmp/net-snmp-config.h)</p>
  
  <blockquote>
    <p>../snmptest1221313123/main.cpp:1:38: fatal error: net-snmp/net-snmp-config.h: No such file or directory
     #include </p>
  </blockquote>
  
  <p>I've read all "add external lib" topics, but it doesn't help to solve this.
  Re-running qmake, re-opening Qt creator, or running qmake -r from terminal doesn't help neither.</p>