libglog.pc: Set Libs.private for static linking

[Arfrever]

This implements suggestion from #57.

Before:

$ pkg-config --libs libglog
-lglog
$ pkg-config --libs --static libglog
-lglog

After:

$ pkg-config --libs libglog
-lglog
$ pkg-config --libs --static libglog
-lglog -lunwind -lpthread -lgflags

By the way, I was also trying get_target_property(some_output_variable glog LINK_LIBRARIES), but it provides raw cmake-style value such as unwind::unwind;-lpthread;gflags which would need complicated parsing and transformation, so the solution used in this pull request seems easier.

[sergiud]

LGTM. Only some nitpicks.

[codecov-commenter]

Codecov Report

Merging #787 (1f5efdf) into master (1883610) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #787   +/-   ##
=======================================
  Coverage   72.72%   72.72%           
=======================================
  Files          17       17           
  Lines        3256     3256           
=======================================
  Hits         2368     2368           
  Misses        888      888           

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[sergiud]

Thanks!