Update bindgen to v0.58

[lupine]

bindgen v0.54 transitively pulls in syntex_syntax v0.58.1, which no longer compiles on Rust v1.41+:

rust-lang/rust#68729

I'm hoping to use libpurple-rust in a rust-native reimplementation of https://code.ur.gs/lupine/purple-plugin-delta , which will need to build against Rust v1.50, so fixing this would be nice.

I fixed a few deprecation warnings in build.rs, but otherwise this seems to be a simple update.

[lupine]

Here's the new output of cargo build on my machine:

$ cargo build
   Compiling memchr v2.3.4
   Compiling libc v0.2.93
   Compiling glob v0.3.0
   Compiling version_check v0.9.3
   Compiling cfg-if v1.0.0
   Compiling proc-macro2 v1.0.26
   Compiling bitflags v1.2.1
   Compiling log v0.4.14
   Compiling unicode-xid v0.2.1
   Compiling regex-syntax v0.6.23
   Compiling unicode-width v0.1.8
   Compiling termcolor v1.1.2
   Compiling humantime v2.1.0
   Compiling vec_map v0.8.2
   Compiling bindgen v0.58.1
   Compiling strsim v0.8.0
   Compiling ansi_term v0.11.0
   Compiling lazy_static v1.4.0
   Compiling shlex v1.0.0
   Compiling rustc-hash v1.1.0
   Compiling lazycell v1.3.0
   Compiling peeking_take_while v0.1.2
   Compiling pkg-config v0.3.19
   Compiling libloading v0.7.0
   Compiling textwrap v0.11.0
   Compiling nom v5.1.2
   Compiling clang-sys v1.2.0
   Compiling aho-corasick v0.7.15
   Compiling quote v1.0.9
   Compiling atty v0.2.14
   Compiling which v3.1.1
   Compiling clap v2.33.3
   Compiling regex v1.4.5
   Compiling cexpr v0.4.0
   Compiling env_logger v0.8.3
   Compiling purple-sys v0.0.5 (.../libpurple-rust)
    Finished dev [unoptimized + debuginfo] target(s) in 24.61s