libpthsem: fix setjmp/longjmp detection

libpthsem: fix setjmp/longjmp detection

libpthsem has been affected since quite a while by build issues, but
occuring only on Nathaniel Roach's autobuilder. The failure message
is:

  error: #error "Unsupported Linux (g)libc version and/or platform"

This message comes from pth_mctx.c file, which implements five methods
for machine context initialization:

  VARIANT 1: THE STANDARDIZED SVR4/SUSv2 APPROACH
  VARIANT 2: THE SIGNAL STACK TRICK
  VARIANT 3: LINUX SPECIFIC JMP_BUF FIDDLING
  VARIANT 4: INTERACTIVE SPECIFIC JMP_BUF FIDDLING
  VARIANT 5: WIN32 SPECIFIC JMP_BUF FIDDLING

The "Unsupported (g)libc version and/or platform" only appears when
"VARIANT 4" is used, since VARIANT 4 only supports a very limited
number of platforms. So when building with Nathaniel's autobuilder,
VARIANT 4 is chosen.

However, when you build libpthsem on some other machine than
Nathaniel's autobuilder, VARIANT 2 is chosen, and works regardless of
the glibc version or architecture.

VARIANT 2 is chosen when:

      !PTH_MCTX_DSP(sjljlx)  &&\
      !PTH_MCTX_DSP(sjljisc) &&\
      !PTH_MCTX_DSP(sjljw32)

On both Nathaniel's autobuilder, and on a different machine, the
PTH_MCTX_MTH macro gives sjlj:

  #define PTH_MCTX_MTH_use PTH_MCTX_MTH_sjlj

However, on a "normal" machine, the PTH_MCTX_DSP macro gives ssjlj:

  #define PTH_MCTX_DSP_use PTH_MCTX_DSP_ssjlj

While on Nathaniel's autobuilder, it gives:

  #define PTH_MCTX_DSP_use PTH_MCTX_DSP_sjljlx

This explains why VARIANT 4 is being used on Nathaniel's autobuilder,
while VARIANT 2 is used when building on other platforms.

The decision of the value for PTH_MCTX_DSP is derived as follows in
configure.ac:

 AC_CHECK_SJLJ(sjlj=yes, sjlj=no, sjlj_type)
 [...]
 elif test ".$sjlj" = .yes; then
    mctx_mth=sjlj
    mctx_dsp=$sjlj_type
 [...]
 AC_DEFINE_UNQUOTED(PTH_MCTX_DSP_use, [PTH_MCTX_DSP_$mctx_dsp], [define for machine context dispatching])

So basically, the value of PTH_MCTX_DSP is $sjlj_type, as returned by
the AC_CHECK_SJLJ autoconf macro, implemented in
acinclude.m4. However, reading this macro is quite informative: it
does a number of tests that are not cross-compile
friendly. Especially, it looks at the kernel version with 'uname -r'
to decide whether the Linux system is braindead or not. If the system
runs a 2.2.x kernel or newer 2.x, or a 3.x kernel, everything is fine,
the system is not braindead, and sjlj_type is set to ssjlj. However,
if the build system runs a 4.x kernel, then it is considered as
braindead, and sjlj_type is set to sjljlx.

And indeed, Nathaniel's autobuilder is running a 4.x kernel, while all
other autobuilders run 2.x or 3.x kernels.

Since for all sane Linux systems, this AC_CHECK_SJLJ macro concludes
that the setjmp/longtmp type is ssjlj, this commit takes the simplest
route of forcing this value, skipping the broken detection.

Note that we're overriding ac_cv_check_sjlj instead of using the
--with-mctx-* options, since the latter do not work properly in the
context of Nathaniel's autobuilder, as the broken cross-compilation
tests continue to cause problems.

Fixes:

  http://autobuild.buildroot.org/results/3dd/3dd66d70c2e36f2d9fb0a0fe01bbdec009d55067/
  and many similar build failures

This patch has been tested by Nathaniel Roach in the context of his
autobuilder instance which was causing the original problem.

Tested-by: Nathaniel Roach <nroach44@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>