[CMake] Is there a way to omit relative path from the name of the comilation output file (<OBJECT>)?

[Alexander Tarnopolsky]

Dear CMake users list,

In our build system we have to compile source files that reside out of
source/build tree. In some cases there are several hundreds of such
"external" sources in a project.

Consider simplified CMakeLists.txt:
===================================
Project (system)

set(SOURCES
/users/gdadmin/PG41/project/src/library/system/AttachDebugger.cc
/users/gdadmin/PG41/project/src/library/system/CPUInfo.cc
DSOInfo.cc
DSOManip.cc
HelpControl.cc
LocateExe.cc
MksTemp.cc
PathManip.cc
PluginLoader.cc
ProcessAffinity.cc
RTTI.cc
Setenv.cc
ShellUtils.cc
Sleep.cc
SysTime.cc
TraceStack.cc
UserInfo.cc
VTControl.cc
Asm/CPUID_c.cc
)

add_library (system SHARED ${SOURCES})
=======================================

The compilation of the first source produces the following output file:
"CMakeFiles/system.dir/users/gdadmin/PG41/project/src/library/system/Att
achDebugger.cc.o"

Imagine a link command where several hundreds of object files are listed
with this relative path...

Thus we are looking for a way to shorten the path. Ideally - we'd like
to generate compiled objects under the same flat-directory regardless of
the source file location.

I understand that ""CMakeFiles/system.dir" part of the relative path to
object files cannot be changed - I hope we can handle this.

But is there a way to change
"/users/gdadmin/PG41/project/src/library/system" part?

I experimented with CMAKE_CXX_COMPILE_OBJECT rule-variable and tried to
replace the <OBJECT> with something else, but apparently the
rule-command is invoked always when <OBJECT> file is missing, probably
the dependency between <OBJECT> and <SOURCE> drives the rule-command.

Of course there is a possibility to compile objects by a custom command
and to manage object's dependencies by a script etc.. However we don't
like to give up on cmake built-in rules for compilation and dependency
tracking.

Thanks in advance for your help,
Kind Regards,
Alex Tarnopolsky
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