Refactor parsing `gcc -E`

[TApplencourt]

$parser.parse(preprocessed_sources_libc)
begin
  preprocessed_sources_libc = $cpp.preprocess
#include <stdint.h>
EOF
rescue
  C::Preprocessor.command = "gcc -E"
  preprocessed_sources_libc = $cpp.preprocess
#include <stdint.h>
EOF
end
$parser.parse(preprocessed_sources_libc.gsub(/^#.*?$/, ''))

Should be a function a re-used