Please fix clang warnings

[yurivict]

Getting these with clang-40:

src/samtools/bgzf.c:247:17: warning: implicit conversion from 'const int' to 'bgzf_byte_t' (aka 'signed char') changes value from 139 to -117 [-Wconstant-conversion]
    buffer[1] = GZIP_ID2;
              ~ ^~~~~~~~
src/samtools/bgzf.c:255:17: warning: implicit conversion from 'const int' to 'bgzf_byte_t' (aka 'signed char') changes value from 255 to -1 [-Wconstant-conversion]
    buffer[9] = OS_UNKNOWN;
              ~ ^~~~~~~~~~
src/pwm_utils.c:11:27: warning: implicitly declaring library function 'strlen' with type 'unsigned long (const char *)' [-Wimplicit-function-declaration]
        int string_size = strlen(argv[1]);
                          ^
lib/bx/bbi/bbi_file.c:8257:32: warning: comparison of unsigned expression < 0 is always false [-Wtautological-compare]
  __pyx_t_11 = ((__pyx_v_start < 0) != 0);
                 ~~~~~~~~~~~~~ ^ ~
src/bunzip/micro-bunzip.c:629:12: warning: assigning to 'unsigned char *' from 'char *' converts between pointers to integer types with different sign [-Wpointer-sign]
                bd->inbuf=inbuf;
                         ^~~~~~