Discourage repeated usage of VARIABLE-PREFIX in PKG_CHECK_MODULES

When the VARIABLE-PREFIX tag is used more than once in PKG_CHECK_MODULES
it can result in later runs reporting success without actually running
pkg-config. This is because PKG_CHECK_MODULES looks for FOO_CFLAGS and
FOO_LIBS from the environment as a way to allow users to override the
test. PKG_CHECK_MODULES also sets these variables, though, and
subsequent calls will simply use these values.

Although there are legitimate ways to use the same VARIABLE-PREFIX
(e.g., they're all under conditionals meaning only one will be used),
document that repeated usage is not recommended.

Freedesktop #3550

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