Changes to support GCC build

[markstock]

It might be my older qt5-qmake, but the code standard (c++2a) doesn't make its way into the compilation command-lines for GCC 9.2. I made it explicit with the following additions to src/vasc.pro:

macx {
    INCLUDEPATH += /usr/local/include
} else {
    # GCC specific configuration
    QMAKE_CXXFLAGS += -std=c++2a
    QMAKE_LFLAGS += -std=c++2a
}

Also, in order to use std::bind in src/jobcontroller.h, I had to #include <functional>.

Finally, GCC wants to explicitly mention pthreads on the link command line, so I modified vasc.pro to have:

LIBS += -L/usr/local/lib -lopenvdb -ltbb -lHalf -lpthread

Now my only stumbling block is the old version of OpenVDB on my system. Let it be known that v4.0.2 is too old.