www: url_handlers: Fix a compiler warning in process_receipt_approval

Due to commit 60d71e0 'www: utils: Make get_{avar,var}() return a const
char *' we now get the following compiler warning in
url_handlers.c::process_receipt_approval()

    url_handlers.c: In function ‘process_receipt_approval’:
    url_handlers.c:1261:18: warning: initialization discards ‘const’
    qualifier from pointer target type [enabled by default]

This is due to storing the return value from get_avar() into a char *
rather then const char *, however, we also then modify the variable, to
inform whether to skip processing or not, so can't just assign it to
a const.

Fix this up by creating a bool skip variable to use rather then
modifying the variable and use that to check whether to skip processing
or not.

Now we can assign the get_avar() variable as a const char *

Signed-off-by: Andrew Clayton <andrew@opentechlabs.co.uk>
www: Makefile: Prettify output and clean up

The main thing is to prettify the output, Whereas before you would get
output like:

    gcc -Wall -std=c99 -O2 -g -Wp,-D_FORTIFY_SOURCE=2 -D_FILE_OFFSET_BITS=64
    -fexceptions -fstack-protector --param=ssp-buffer-size=4 -fPIE -c
    utils.c -I../../libctemplate `pkg-config --cflags glib-2.0` `pkg-config
    --cflags gmime-2.0` `mysql_config --cflags`

for each C file. We now take a leaf from the Kernel and produce output
like:

    CC	utils.o

This grealty reduces the clutter and makes compiler warnings easier to
see.

Also set an objects variable that contains all the .o files to negate
the need to list them all out several times.

Signed-off-by: Andrew Clayton <andrew@opentechlabs.co.uk>