qmake: Use flags from pkg-config if they exist, otherwise use defaults

Question

In my setup, I want to support both system-wide Qt installations and custom Qt installations. I can use pkg-config to get the correct compile and link flags for my system-wide installation:

CONFIG += link_pkgconfig
PKGCONFIG += Qt5Core

However, if pkg-config cannot find Qt5Core, the build will fail with Project ERROR: Qt5Core development package not found.

Instead of failing, I want to set reasonable defaults (e.g. /usr/local/qt5). It should achieve the following:

if pkg-config can find Qt5Core {
    PKGCONFIG += Qt5Core
} else {
    INCLUDEPATH += /usr/local/qt5/
    LIBS += -lQt5Core
}

How can I accomplish this in my project configuration?

Answer 1

There is a qmake function for this: http://doc.qt.io/qt-5/qmake-test-function-reference.html#packagesexist-packages

packagesExist(Qt5Core) {
    PKGCONFIG += Qt5Core
} else {
    INCLUDEPATH += /usr/local/qt5/
    LIBS += -lQt5Core
}