QT -= core
QT -= gui

CONFIG += c++14 console
CONFIG -= app_bundle

DEFINES += INCLUDE_ME

SOURCES += main.cpp

HEADERS += \
    include_me.h

#if defined(INCLUDE_ME)
#include "include_me.h"
#else
#include "dont_include_me.h"
#endif

int main()
{
    return 0;
}

...
####### Compile

main.o: ../testcpp06/main.cpp ../testcpp06/include_me.h
	$(CXX) -c $(CXXFLAGS) $(INCPATH) -o main.o ../testcpp06/main.cpp
...

...
####### Compile

main.o: ../testcpp06/main.cpp ../testcpp06/include_me.h \
		dont_include_me.h
	$(CXX) -c $(CXXFLAGS) $(INCPATH) -o main.o ../testcpp06/main.cpp
...