configure: Omit -O2 from CFLAGS when configuring with --enable-coverage.

configure: Omit -O2 from CFLAGS when configuring with --enable-coverage.

CFLAGS follows OVS_CFLAGS in the compiler command line, and should, so that
the user can override any automatically determined compiler options.  That
means that the -O0 that the code here added to OVS_CFLAGS didn't really
have any effect since CFLAGS by default includes "-O2".  However, we do
really want to disable optimizations (because the coverage reports are hard
to interpret with optimizations), so this commit changes the configure
script to edit out -O2 from CFLAGS, leaving any other default or
user-specified options.

Reported-by: Mijo Safradin <mijo@linux.vnet.ibm.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>