Make gflags an IMPORTED GLOBAL library

[amyspark]

See:

• [gflags]: Fix targets gflags::* not being globally visible microsoft/vcpkg#20011

• https://stackoverflow.com/questions/45401212/how-to-make-imported-target-global-afterwards/48390363#48390363

Fixes #1197

[mo271]

Thanks for the contribution! Could you add your name to the AUTHORS file (keeping alphabetical order..)?

[mo271]

Thanks for modifying AUTHORS! This check is green now.

Change applied

[sboukortt]

Thanks!

[jonsneyers]

This broke something for the Release linux x86_64 static build:

[288/310] Linking CXX executable tools/cjxl_ng
FAILED: tools/cjxl_ng 
: && /usr/bin/clang++ -ffunction-sections -fdata-sections -fno-rtti -funwind-tables -Xclang -mrelax-all -Xclang -mconstructor-aliases -fno-omit-frame-pointer -O3 -DNDEBUG -O2 -Wl,--gc-sections -Wl,--no-export-dynamic -static -static-libgcc -static-libstdc++    -fPIE -pie tools/CMakeFiles/cjxl_ng.dir/cjxl_ng_main.cc.o -o tools/cjxl_ng  lib/libjxl.a  lib/libjxl_extras_dec-static.a  lib/libjxl_threads.a  third_party/highway/libhwy.a  /usr/lib/x86_64-linux-gnu/libgflags.so.2.2.2  tools/box/libbox.a  lib/libjxl.a  lib/libjxl_extras-static.a  lib/libjxl_threads.a  tools/libjxl_tool.a  lib/libjxl_extras_dec-static.a  /usr/lib/x86_64-linux-gnu/libgif.a  /usr/lib/x86_64-linux-gnu/libjpeg.a  /usr/lib/x86_64-linux-gnu/libpng.a  /usr/lib/x86_64-linux-gnu/libz.a  third_party/sjpeg/libsjpeg.a  lib/libjxl.a  third_party/highway/libhwy.a  third_party/brotli/libbrotlidec-static.a  -lpthread  -Wl,--whole-archive  -Wl,--no-whole-archive  third_party/brotli/libbrotlienc-static.a  third_party/brotli/libbrotlicommon-static.a  -lm && :
clang: warning: argument unused during compilation: '-pie' [-Wunused-command-line-argument]
/usr/bin/ld: attempted static link of dynamic object `/usr/lib/x86_64-linux-gnu/libgflags.so.2.2.2'
clang: error: linker command failed with exit code 1 (use -v to see invocation)

[mo271]

This broke something for the Release linux x86_64 static build:

[288/310] Linking CXX executable tools/cjxl_ng
FAILED: tools/cjxl_ng 
: && /usr/bin/clang++ -ffunction-sections -fdata-sections -fno-rtti -funwind-tables -Xclang -mrelax-all -Xclang -mconstructor-aliases -fno-omit-frame-pointer -O3 -DNDEBUG -O2 -Wl,--gc-sections -Wl,--no-export-dynamic -static -static-libgcc -static-libstdc++    -fPIE -pie tools/CMakeFiles/cjxl_ng.dir/cjxl_ng_main.cc.o -o tools/cjxl_ng  lib/libjxl.a  lib/libjxl_extras_dec-static.a  lib/libjxl_threads.a  third_party/highway/libhwy.a  /usr/lib/x86_64-linux-gnu/libgflags.so.2.2.2  tools/box/libbox.a  lib/libjxl.a  lib/libjxl_extras-static.a  lib/libjxl_threads.a  tools/libjxl_tool.a  lib/libjxl_extras_dec-static.a  /usr/lib/x86_64-linux-gnu/libgif.a  /usr/lib/x86_64-linux-gnu/libjpeg.a  /usr/lib/x86_64-linux-gnu/libpng.a  /usr/lib/x86_64-linux-gnu/libz.a  third_party/sjpeg/libsjpeg.a  lib/libjxl.a  third_party/highway/libhwy.a  third_party/brotli/libbrotlidec-static.a  -lpthread  -Wl,--whole-archive  -Wl,--no-whole-archive  third_party/brotli/libbrotlienc-static.a  third_party/brotli/libbrotlicommon-static.a  -lm && :
clang: warning: argument unused during compilation: '-pie' [-Wunused-command-line-argument]
/usr/bin/ld: attempted static link of dynamic object `/usr/lib/x86_64-linux-gnu/libgflags.so.2.2.2'
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Seems to be fixed here #1364