HEADERS
is supposed to list the header files of your own project, just like SOURCES
lists the source files.
If you want to include external header files, you should add their folders to INCLUDEPATH
instead:
INCLUDEPATH += inc
The following .pro file works perfectly in Qt Creator 2.5.0, Qt 4.6.1:
QT += core
QT -= gui
TARGET = test
CONFIG += console
CONFIG -= app_bundle
TEMPLATE = app
SOURCES += \
src/1.cpp
INCLUDEPATH += inc
Try that exact .pro file, give it 3 seconds to parse and tell me if it worked.