Pycairo and Pygtk include Python headers and use -std=c99 (or -std=c9x).  On Solaris 10, Python 2.5.1's pyconfig.h defines _XOPEN_SOURCE=500.  Solaris's <sys/feature_tests.h> balks when a C99 compiler is used in combination with _XOPEN_SOURCE=500.  Removing the -std=c99 (or c9x) allows the build to complete.

I recently commented on a similar bug in Pygobject (#339924), and Gustavo Carneiro conditionally removed -std=c9x on Solaris, which fixed the problem for me in Pygobject.  However, perhaps -std=c99 (or c9x) is not needed anywhere anymore.  Pygtk's ChangeLog-pre-2-0 includes:

2002-01-31  James Henstridge  <james@daa.com.au>

        * configure.in: increment version number of package and
        requirements.  Change -ansi to -std=c9x to work arround potential
        problems with gcc 2.95.x.

Does GCC 2.95 still need to be supported?  (Especially since the minimum Python version is from 2003?)  If not, perhaps -std=c99 could be droppend entirely.

I have Solaris 10 11/06 with the stock GCC (3.4.3) and Sun linker.  I built and installed Python 2.5.1 (my config.log says "./configure --enable-shared").  Here's the error when building Pycairo:

gcc -DHAVE_CONFIG_H -I. -I.. -I/usr/local/include/python2.5 -I/usr/local/include/cairo -I/usr/sfw/include -I/usr/sfw/include/freetype2 -I/usr/include/libpng12 -g -O2 -std=c99 -Wall -fno-strict-aliasing -MT _cairo_la-pycairo-context.lo -MD -MP -MF .deps/_cairo_la-pycairo-context.Tpo -c pycairo-context.c  -fPIC -DPIC -o .libs/_cairo_la-pycairo-context.o
In file included from /usr/include/limits.h:18,
                 from /usr/local/include/python2.5/Python.h:18,
                 from pycairo-context.c:32:
/usr/include/sys/feature_tests.h:332:2: #error "Compiler or options invalid for pre-UNIX 03 X/Open applications         and pre-2001 POSIX applications"

Some minimal test programs:

$ cat test.c
#define _XOPEN_SOURCE 500
#include <unistd.h>
$ gcc -std=c99 -c test.c
In file included from /usr/include/unistd.h:18,
                 from test.c:3:
/usr/include/sys/feature_tests.h:332:2: #error "Compiler or options invalid for pre-UNIX 03 X/Open applications         and pre-2001 POSIX applications"

$ cat > pytest.c
#include <Python.h>
$ gcc -I/usr/local/include/python2.5 -c pytest.c
$ gcc -std=c99 -I/usr/local/include/python2.5 -c pytest.c
In file included from /usr/include/limits.h:18,
                 from /usr/local/include/python2.5/Python.h:18,
                 from pytest.c:1:
/usr/include/sys/feature_tests.h:332:2: #error "Compiler or options invalid for pre-UNIX 03 X/Open applications         and pre-2001 POSIX applications"