Correctly set position-independent code

[mjcarroll]

This was broken by gazebosim/gz-cmake#399, since we dropped the gazebo-specific flag, we can use native CMake mechanism for marking this library position-independent.

Without this setting, warning messages like this would appear because we were no longer adding the -fPIC flag to static libraries.

--- stderr: gz-rendering7                                                                         
/usr/bin/ld: ../../lib/libterra.a(OgreHlmsTerra.cpp.o): warning: relocation against `_ZN4Ogre11PbsProperty14LegacyMathBrdfE' in read-only section `.text'
/usr/bin/ld: ../../lib/libterra.a(OgreHlmsTerra.cpp.o): relocation R_X86_64_PC32 against symbol `_ZTVN4Ogre9HlmsTerraE' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: final link failed: bad value
collect2: error: ld returned 1 exit status

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (451e34e) 77.62% compared to head (191765d) 77.62%.

Additional details and impacted files

@@              Coverage Diff               @@
##           gz-rendering7     #950   +/-   ##
==============================================
  Coverage          77.62%   77.62%           
==============================================
  Files                170      170           
  Lines              14726    14726           
==============================================
  Hits               11431    11431           
  Misses              3295     3295           

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[mjcarroll]

Window failures are existing here.