pkg-config: Add gnu89 fixes Lion + Clang
pkg-config: Add gnu89 fixes Lion + Clang

pkg-config append to cflags -std=gnu89 on Lion fixes duplicate
symbols linking libglib when compiling with Clang-3.0 from
XCode-4.2 (4D199).  Add make check to confirm success, given the
critical nature of the software.  The flag is appended only on
Lion, though it successfully tests on Snow Leopard in 64bit, too.
EDIT: checks if lion and if clang before switching to gnu89.

Closes Homebrew/homebrew#8631.

Signed-off-by: Charlie Sharpsteen <828d338a9b04221c9cbe286f50cd389f68de4ecf@sharpsteen.net>

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