pv: support ccache builds

pv: support ccache builds

When using ccache TARGET_CC is something like
    ccache gcc

This causes problems in the pv build which attempts to
override LD because the command ends up being
    make [...] LD=/tools/ccache /tools/gcc LDFLAGS="[...]

As a result, during the build phase it attempts to build
/tools/gcc which succeeds by doing nothing:
    make[1]: Nothing to be done for `/tools/gcc'.

and during the install phase you get the real build which
errors out on the LD error this snippet was attempting to
fix:
    ld -r -o src/library.o src/library/getopt.o \
        src/library/gettext.o
    ld: src/library/gettext.o: Relocations in generic ELF \
        (EM: 40)
    src/library/gettext.o: error adding symbols: File in \
        wrong format

Signed-off-by: Andrew Ruder <andrew.ruder@elecsyscorp.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>