gflags: build as shared library
gflags: build as shared library

- By default, newer versions of gflags that use CMake, build a static
  library, whereas previously (e.g. v2.0) it was built as a shared
  library.
- Building gflags statically causes issues when glog is built as a
  shared library (its default option).  Specifically, the gflags defined
  by glog will not be visible unless glog appears explicitly prior to
  gflags in the link step for an application (i.e. there are undefined
  symbols from gflags that need to be resolved).
- In fact, on Linux the linker will refuse to build glog (0.3.4) as a
  shared library if gflags (2.1.2) is built as a static library due to
  gflags not being position independent.

Closes Homebrew/homebrew#41151.

Signed-off-by: Dominyk Tiller <53e438f55903875d07efdd98a8aaf887e7208dd3@gmail.com>

cblecker/homebrew-core,cblecker/homebrew-core,jdubois/homebrew-core,jabenninghoff/homebrew-core,bigbes/homebrew-core,jvillard/homebrew-core,mbcoguno/homebrew-core,lasote/homebrew-core,lembacon/homebrew-core,jvehent/homebrew-core,mvbattista/homebrew-core,sjackman/homebrew-core,bigbes/homebrew-core,chrisfinazzo/homebrew-core,jvillard/homebrew-core,mvitz/homebrew-core,git-lfs/homebrew-core,lembacon/homebrew-core,lemzwerg/homebrew-core,ShivaHuang/homebrew-core,tkoenig/homebrew-core,ilovezfs/homebrew-core,straxhaber/homebrew-core,zmwangx/homebrew-core,ylluminarious/homebrew-core,reelsense/homebrew-core,j-bennet/homebrew-core,mahori/homebrew-core,reelsense/homebrew-core,sdorra/homebrew-core,mfikes/homebrew-core,tkoenig/homebrew-core,lasote/homebrew-core,battlemidget/homebrew-core,jabenninghoff/homebrew-core,lemzwerg/homebrew-core,mfikes/homebrew-core,wolffaxn/homebrew-core,mbcoguno/homebrew-core,ShivaHuang/homebrew-core,aabdnn/homebrew-core,rwhogg/homebrew-core,uyjulian/homebrew-core,passerbyid/homebrew-core,kunickiaj/homebrew-core,filcab/homebrew-core,git-lfs/homebrew-core,kuahyeow/homebrew-core,j-bennet/homebrew-core,adamliter/homebrew-core,mahori/homebrew-core,zmwangx/homebrew-core,Moisan/homebrew-core,adam-moss/homebrew-core,maxim-belkin/homebrew-core,JCount/homebrew-core,edporras/homebrew-core,dsXLII/homebrew-core,rwhogg/homebrew-core,uyjulian/homebrew-core,nbari/homebrew-core,zyedidia/homebrew-core,nandub/homebrew-core,mvitz/homebrew-core,phusion/homebrew-core,stgraber/homebrew-core,grhawk/homebrew-core,Homebrew/homebrew-core,ylluminarious/homebrew-core,grhawk/homebrew-core,gserra-olx/homebrew-core,LinuxbrewTestBot/homebrew-core,bcg62/homebrew-core,battlemidget/homebrew-core,maxim-belkin/homebrew-core,edporras/homebrew-core,nbari/homebrew-core,stgraber/homebrew-core,lembacon/homebrew-core,DomT4/homebrew-core,makigumo/homebrew-core,mbcoguno/homebrew-core,LinuxbrewTestBot/homebrew-core,aren55555/homebrew-core,zyedidia/homebrew-core,moderndeveloperllc/homebrew-core,aabdnn/homebrew-core,chrisfinazzo/homebrew-core,sjackman/homebrew-core,BrewTestBot/homebrew-core,fvioz/homebrew-core,wolffaxn/homebrew-core,Linuxbrew/homebrew-core,bcg62/homebrew-core,fvioz/homebrew-core,FinalDes/homebrew-core,mvitz/homebrew-core,phusion/homebrew-core,spaam/homebrew-core,adamliter/homebrew-core,makigumo/homebrew-core,filcab/homebrew-core,nandub/homebrew-core,bcg62/homebrew-core,robohack/homebrew-core,andyjeffries/homebrew-core,ilovezfs/homebrew-core,jvehent/homebrew-core,smessmer/homebrew-core,dsXLII/homebrew-core,adam-moss/homebrew-core,kuahyeow/homebrew-core,Linuxbrew/homebrew-core,jdubois/homebrew-core,sdorra/homebrew-core,passerbyid/homebrew-core,robohack/homebrew-core,jvillard/homebrew-core,jdubois/homebrew-core,bfontaine/homebrew-core,mvbattista/homebrew-core,forevergenin/homebrew-core,andyjeffries/homebrew-core,moderndeveloperllc/homebrew-core,aren55555/homebrew-core,kunickiaj/homebrew-core,Moisan/homebrew-core,bfontaine/homebrew-core,JCount/homebrew-core,BrewTestBot/homebrew-core,wolffaxn/homebrew-core,FinalDes/homebrew-core,smessmer/homebrew-core,gserra-olx/homebrew-core,forevergenin/homebrew-core,straxhaber/homebrew-core,spaam/homebrew-core,DomT4/homebrew-core,Homebrew/homebrew-core