Meson not fully respecting -I/-L inside CLFAGS/LDFLAGS

It's not unusual for projects to have a custom ` contrib` tree that contains bundled third party dependencies (+ their dependencies) to build these inside a custom tree ([example](https://git.videolan.org/?p=vlc.git;a=tree;f=contrib;h=f4e4131290921b7af736b0cb04c3a7c23be8ada9;hb=HEAD)). This is usually done by adding extra cflags and ldflags to set the correct `-L` and `-I` so that projects can find their own dependencies.

With autotools, for example, this works fine - doing the equivalent of `cc.find_library()` or `cc.get_define()` in autotools respects the set `-L` and `-I` set via e.g. the environment. With meson, however, this same approach does not work. Even with something like `LDFLAGS="-L/path/to/contrib/build/lib" CFLAGS="$LDFLAGS -I/path/to/contrib/build/include" meson --prefix /path/to/contrib/build`, meson fails to find a simple library or header, as in the following example:

```shell
$ cd /path/to/contrib && tree
.
├── bar.autotools
│   └── configure.ac
├── bar.meson
│   └── meson.build
└── build
├── include
│   └── foo.h
└── lib
└── libfoo.so

5 directories, 4 files
```

`bar.meson/meson.build`:
```meson
project('bar', 'c')

cc = meson.get_compiler('c')
foo = cc.find_library('foo', required: true)
ver = cc.get_define('FOO_VERSION', prefix: '#include ').to_int()
```

To work around the issue in this example, I could do something like this:

```meson
project('bar', 'c')

prefix = get_option('prefix')
prefix_lib = join_paths(prefix, get_option('libdir'))
prefix_inc = include_directories(join_paths(prefix, get_option('includedir')))

foo = cc.find_library('libfoo.so', dirs: prefix_lib, required: true)
ver = cc.get_define('FOO_VERSION',
prefix: '#include ',
include_directories: prefix_inc,
).to_int()
```

Alternatively, I could expose the equivalent of `prefix_lib` and `prefix_inc` directly as string options that the user can provide.

However both of these approaches rely on either patching the meson.build of every project, or somehow communicating a change like this upstream to every one of the projects. An ideal solution would consist of some way to explicitly add extra lib/include search paths at configure time, either via CFLAGS/LDFLAGS as before, explicitly via some meson built-in option that we can set via `-D`, or implicitly via `--prefix`.

Reproduce the example from bar.meson/meson.build using the shown CFLAGS and LDFLAGS, then compare cc.find_library() and cc.get_define() with the prefix-based workaround. Review how configure-time search paths are handled and define a supported approach that lets both checks find the bundled header and library without patching each project.

Written by the indexing model from the issue text.