Use _GNU_SOURCE macro on Android

[jimmy-park]

I've got the following error while building c-ares on Android.

FAILED: _deps/c-ares-source-build/src/lib/CMakeFiles/c-ares.dir/Debug/event/ares_event_wake_pipe.c.o 
/Users/jimmy.park/Library/Android/sdk/ndk/27.2.12479018/toolchains/llvm/prebuilt/darwin-x86_64/bin/clang --target=armv7-none-linux-androideabi33 --sysroot=/Users/jimmy.park/Library/Android/sdk/ndk/27.2.12479018/toolchains/llvm/prebuilt/darwin-x86_64/sysroot -DCARES_BUILDING_LIBRARY -DHAVE_CONFIG_H=1 -DCMAKE_INTDIR=\"Debug\" -I/Users/jimmy.park/test-c-ares/build/android-arm/_deps/c-ares-source-build -I/Users/jimmy.park/test-c-ares/build/.cache/c-ares-source/b6ff14176cd38f824ca7aa8b1acab422ed5556a6 -I/Users/jimmy.park/test-c-ares/build/.cache/c-ares-source/b6ff14176cd38f824ca7aa8b1acab422ed5556a6/include -I/Users/jimmy.park/test-c-ares/build/.cache/c-ares-source/b6ff14176cd38f824ca7aa8b1acab422ed5556a6/src/lib -I/Users/jimmy.park/test-c-ares/build/.cache/c-ares-source/b6ff14176cd38f824ca7aa8b1acab422ed5556a6/src/lib/include -g -DANDROID -fdata-sections -ffunction-sections -funwind-tables -fstack-protector-strong -no-canonical-prefixes -D_FORTIFY_SOURCE=2 -march=armv7-a -mthumb -Wformat -Werror=format-security  -Wall -Wextra -Waggregate-return -Wcast-align -Wcast-qual -Wconversion -Wdeclaration-after-statement -Wdouble-promotion -Wfloat-equal -Wformat-security -Winit-self -Wmissing-braces -Wmissing-declarations -Wmissing-format-attribute -Wmissing-include-dirs -Wmissing-prototypes -Wnested-externs -Wold-style-definition -Wpacked -Wpedantic -Wpointer-arith -Wredundant-decls -Wshadow -Wsign-conversion -Wstrict-overflow -Wstrict-prototypes -Wundef -Wunreachable-code -Wunused -Wvariadic-macros -Wvla -Wwrite-strings -Werror=implicit-int -Werror=implicit-function-declaration -Werror=partial-availability -Qunused-arguments -Wno-long-long -fdiagnostics-color=always -fcolor-diagnostics -fno-limit-debug-info  -fno-omit-frame-pointer -O0 -std=c99 -fPIC -fcolor-diagnostics -w -MD -MT _deps/c-ares-source-build/src/lib/CMakeFiles/c-ares.dir/Debug/event/ares_event_wake_pipe.c.o -MF _deps/c-ares-source-build/src/lib/CMakeFiles/c-ares.dir/Debug/event/ares_event_wake_pipe.c.o.d -o _deps/c-ares-source-build/src/lib/CMakeFiles/c-ares.dir/Debug/event/ares_event_wake_pipe.c.o -c /Users/jimmy.park/test-c-ares/build/.cache/c-ares-source/b6ff14176cd38f824ca7aa8b1acab422ed5556a6/src/lib/event/ares_event_wake_pipe.c
/Users/jimmy.park/test-c-ares/build/.cache/c-ares-source/b6ff14176cd38f824ca7aa8b1acab422ed5556a6/src/lib/event/ares_event_wake_pipe.c:68:7: error: call to undeclared function 'pipe2'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
   68 |   if (pipe2(p->filedes, O_NONBLOCK | O_CLOEXEC) != 0) {
      |       ^
/Users/jimmy.park/test-c-ares/build/.cache/c-ares-source/b6ff14176cd38f824ca7aa8b1acab422ed5556a6/src/lib/event/ares_event_wake_pipe.c:68:7: note: did you mean 'pipe'?
/Users/jimmy.park/Library/Android/sdk/ndk/27.2.12479018/toolchains/llvm/prebuilt/darwin-x86_64/sysroot/usr/include/unistd.h:275:5: note: 'pipe' declared here
  275 | int pipe(int __fds[_Nonnull 2]);
      |     ^
1 error generated.

It seems like the _GNU_SOURCE macro should be defined because of the pipe2 function.

[bradh352]

Probably be better to just add an OR to the CMAKE_SYSTEM_NAME STREQUAL "Linux" so both Linux and Android use the same flags as it looks like features.h in android talks about those other flags being relevant too. Its possible we've excluded some functionality on Android due to not having those set when building with CMake.