bump gl_IGNORE_UNUSED_LIBRARIES up in the order of things

[Mike Frysinger]

i'm not really familiar with how gnulib is integrated, but the generated 
m4/gnulib-comp.m4 file makes a call to gl_IGNORE_UNUSED_LIBRARIES to add fun 
flags like -Wl,--as-needed to LDFLAGS.  unfortunately, this call happens 
somewhat late in the gnulib m4 process when it should come as soon as 
possible.  reason is that these LDFLAGS can change the results of some of the 
tests which detect external libraries.

for example, consider a gcc flag which pulls in libs which are dynamically 
linked to librt.  the code which detects whether an external -lrt is needed 
would be given back a result that no, no it does not.  but when you 
add -Wl,--as-needed in, the implicit DT_NEEDED for librt.so is suddenly 
irrelevant.  if the LDFLAGS were set as soon as possible, this would have 
been detected properly.

due to my unfamiliarity with gnulib and how that m4 gets generated, i cant 
suggest a fix here :/
-mike

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