use AC_OPENMP if available (autoconf >= 2.62)

[su_v]

Current situation:
Inkscape uses the external macro AX_OPENMP to detect whether the current compiler supports OpenMP.

While this macro seems to work as expected on Linux, it fails to detect OpenMP support of Apple's GCC-4.2 and llvm-gcc-4.2 [1].
Checking for OpenMP support succeeds (even on OS X) if using AC_OPENMP provided by current autoconf versions (>= 2.62) [2].

Proposed solution (patch):
Add test and use AC_OPENMP if available, else resort to external macro AX_OPENMP.

This allows to build Inkscape with OpenMP support (multi-threaded rendering of filter effects) on OS X systems using a current autoconf version and Apple's GCC 4.2 / llvm-gcc-4.2 compilers. The patch should not negatively affect older build systems which might still use autoconf < 2.62.

Patch tested with Inkscape 0.48+devel r11261 using
- autoconf 2.68, GCC 4.2 (Mac OS X 10.5.8, 32bit Intel)
- autoconf 2.68, llvm-gcc-4.2 (OS X 10.7.2, 64bit)

---
[1] <http://thread.gmane.org/gmane.comp.graphics.inkscape.devel/34490/focus=34491>
[2] <http://lists.gnu.org/archive/html/autotools-announce/2008-04/msg00002.html>

[Jon A. Cruz (jon-joncruz)]

Patch seems to work on OS X 10.6.8, but when executing the filter code appears to go into an infinite loop. Autoconf detection was good, though.

[su_v (suv-lp)]

> but when executing the filter code appears to go into an infinite loop

Every time? I manage to produce such hangs (not consistently though) when e.g. quickly clicking on the canvas to select something while screen redrawing is still in process with a complex filter example, but basic rendering for example of the 'share/examples/filters.svg' file is ok, at whatever zoom level I choose.

Overall - in my tests on two different systems - rendering performance is noticeably better with OpenMP support enable (despte occasional hangs):

Tests with optimized (-O2) builds run on
- Mac OS X 10.5.8 i386 (Apple GCC 4.2.1)
- OS X 10.7.2 (Apple llvm-gcc-4.2)

I'd still vote for committing the change (unless it breaks linux builds) - we won't find out whether Inkscape can make use of the new multi-core features on OS X unless we enable it in trunk builds to allow testing.

Using autoconf's AC_OPENMP macro also allows to disable OpenMP support completely with './configure --disable-openmp' (which could be recommended for release builds on OS X if any version of Apple's gcc-4.2 turns out to be too unstable with openmp enabled):

$ ./configure --help | grep OpenMP
  --disable-openmp do not use OpenMP
$

[Alex Valavanis (valavanisalex)]

Updated patch committed in lp:inkscape r12157+12158. The new configure.ac specifies autoconf 2.64 as a prerequisite, so there's no longer any need for the fallback macro.

[su_v (suv-lp)]

OpenMP disabled for regular OS X builds using 'packaging/macosx/osx-build.sh' in r12160 for now - anyone interested in testing can remove the configure option in the script to compile builds with multi-threading enabled, or build without the help of the packaging script.