Glibc build generates header files to define constants from special .sym
files. If a .sym file includes the same header file which it generates,
it leads to circular dependency which may lead to build hang on a
many-core machine. Define GEN_AS_CONST_HEADERS when generating header
files to avoid circular dependency.
<tcb-offsets.h> is needed for i686 and it isn't needed for x86-64 at
least since glibc 2.23.
Tested on i686 and x86-64.
[BZ #22792]
* Makerules ($(common-objpfx)%.h): Pass -DGEN_AS_CONST_HEADERS
to $(CC).
* sysdeps/unix/sysv/linux/i386/lowlevellock.h: Include
<tcb-offsets.h> only if GEN_AS_CONST_HEADERS isn't defined.
* sysdeps/unix/sysv/linux/x86_64/lowlevellock.h: Don't include
<tcb-offsets.h>.
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