add _package_pre check to C library generation
add _package_pre check to C library generation

if there is no package prefix, the old behavior would result in an underscore
'_' being erroneously prefixed to generated source code filenames. new behavior
will only concatenate the package prefix if the LCM message definition file in
question has specified a package.

lcm-proj/lcm,lcm-proj/lcm,lcm-proj/lcm,lcm-proj/lcm,lcm-proj/lcm,lcm-proj/lcm,lcm-proj/lcm,lcm-proj/lcm