maint: turn on gcc logical-op checking

maint: turn on gcc logical-op checking

This would have detected the bug in commit 38ad33931 (Aug 09), which
we missed until commit f828ca35 (Jul 10); over 11 months later.

However, on Fedora 13, it also triggers LOTS of warnings from
the libcurl-devel header for two files:

esx/esx_vi.c: In function 'esxVI_CURL_Perform':
esx/esx_vi.c:232: error: logical '&&' with non-zero constant will always evaluate as true [-Wlogical-op]
esx/esx_vi.c:232: error: logical '&&' with non-zero constant will always evaluate as true [-Wlogical-op]
esx/esx_vi.c:232: error: logical '&&' with non-zero constant will always evaluate as true [-Wlogical-op]
...
xenapi/xenapi_driver.c: In function 'call_func':
xenapi/xenapi_driver.c:1872: error: logical '&&' with non-zero constant will always evaluate as true [-Wlogical-op]
xenapi/xenapi_driver.c:1872: error: logical '&&' with non-zero constant will always evaluate as true [-Wlogical-op]
xenapi/xenapi_driver.c:1872: error: logical '&&' with non-zero constant will always evaluate as true [-Wlogical-op]
...

libcurl allows to disable the type-checking code that triggers those
warnings, along with the reduction in type-safety of calls to some
libcurl functions. I figure this is worth the improved compiler
checking throughout the rest of libvirt.

* acinclude.m4 (--enable-compile-warnings=error): Add -Wlogical-op.
* configure.ac: Add -DCURL_DISABLE_TYPECHECK to LIBCURL_CFLAGS to
avoid compilation warning.

Suggested by Daniel P. Berrange.
Tweaked by Matthias Bolte.