configure: Fix the pkg-config check for pango
configure: Fix the pkg-config check for pango

It looks like the then-clause of the AS_IF macro needs to be in square
brackets otherwise the configure script gets generated wrong and you
get this output when you run it:

configure: line 17339: COGL_PANGO_DEP_CFLAGS: command not found
configure: line 17340: C: command not found
configure: line 17341: COGL_PANGO_DEP_LIBS: command not found
configure: line 17342: linker: command not found

Reviewed-by: Robert Bragg <12e9293ec6b30c7fa8a0926af42807e929c1684f@linux.intel.com>

gcampax/cogl,Distrotech/cogl,djdeath/cogl-android,Distrotech/cogl,Distrotech/cogl,djdeath/cogl-android,Distrotech/cogl,djdeath/cogl,djdeath/cogl,djdeath/cogl,djdeath/cogl-android,gcampax/cogl,gcampax/cogl,gcampax/cogl