Reproduce as follows:

Create a .pro file with the following contents:

TEMPLATE = lib
CONFIG += staticlib
SOURCES += all.c

The contents of "all.c" are irrelevant, you may use an empty file.

The build will always fail until you rename all.c to something else.

An error comes from a garbage additional command, for instance:

Using SOURCE.c the following compile output is shown:

gcc -c -pipe -g -fPIC -Wall -W -D_REENTRANT -DQT_QML_DEBUG -I../all_c_test -I. -I../../qt594debug/mkspecs/linux-g++ -o SOURCE.o ../all_c_test/SOURCE.c
rm -f liball_c_test.a
ar cqs liball_c_test.a SOURCE.o

Using all.c we get the same output plus an additional "gcc" call:

gcc -c -pipe -g -fPIC -Wall -W -D_REENTRANT -DQT_QML_DEBUG -I../all_c_test -I. -I../../qt594debug/mkspecs/linux-g++ -o all.o ../all_c_test/all.c
rm -f liball_c_test.a
ar cqs liball_c_test.a all.o
gcc all.o Makefile liball_c_test.a -o all

it looks like the makefile has an "all" directive that collides with the name of the source OR object file all.c/all.o somehow.

If the name "all" can't be used because when using qmake this should be made clear in the build output, now we get a "collect2: error: ld returned 1 exit status" message.