isolationtester and 'specs' subdirectory

Hi all,
I have a Debian machine with gcc 4.7.2-5 where make check-world fails
in the isolation check, like so:

...
make[2]: Leaving directory `/home/josh/src/postgresql/src/test/regress'
make -C isolation check

[snip]

gcc -O2 -Wall -Wmissing-prototypes -Wpointer-arith
-Wdeclaration-after-statement -Wendif-labels
-Wmissing-format-attribute -Wformat-security -fno-strict-aliasing
-fwrapv -fexcess-precision=standard -g -I.
-I../../../src/interfaces/libpq -I./../regress -I../../../src/include
-D_GNU_SOURCE -c -o isolation_main.o isolation_main.c
gcc: error: ./specs: Is a directory
make[2]: *** [isolation_main.o] Error 1
...

I eventually tracked down the cause of this failure to a trailing ':'
in my $LIBRARY_PATH, which causes gcc to look inside the current
directory for a 'specs' file [1] among other things. Although I
probably don't need that trailing ':', it seems like we should avoid
naming this directory 'specs' nonetheless to avoid confusion with gcc.

Renaming the 'specs' directory to something like 'isolation_specs' and
adjusting isolation_main.c accordingly lets me pass `make
check-world`. Proposed patch attached.

Josh

[1] http://gcc.gnu.org/ml/gcc-help/2010-05/msg00292.html