build: Use Meson’s module for building .pc files

Using the module will prevent trivial bugs caused by incorrect string concatenation.

This is just a literal translation. In the future we might rely on Meson auto-detecting fields from library declaration.

Fixes: #3517 (closed)

Compared the pkg-config files with Meld and there are some minor differences including:

• Requires are comma-separated
• cairo is removed from Requires.private by Meson’s deduplication.
• the space in targets is escaped
• exec_prefix=${prefix} is missing as it is not really used with Meson
• Libs.private: is not added when gdk_extra_libs is empty
• whitespace changes

I do not expect any issues from these except maybe the targets escaping.