lvm2: do not override CC and similar variables

Since lvm2 uses autoconf, there's no point in overriding CC and other
variables at build time. This causes build breakage as CFLAGS aren't
used at link time, therefore --sysroot lacks, which breaks the build
with external toolchains.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>