[BUG] Cannot compile libdatadog common.h on gcc 4.9.2

[ivoanjo]

Describe the bug

While upgrading the Ruby profiler to use libdatadog 0.8.0, I ran into the following issue:

In file included from /usr/local/bundle/gems/libdatadog-0.8.0.1.0-x86_64-linux/vendor/libdatadog-0.8.0/x86_64-linux/libdatadog-x86_64-unknown-linux-gnu/lib/pkgconfig/../../include/datadog/profiling.h:11:0,
                 from ../../../../ext/ddtrace_profiling_native_extension/collectors_stack.h:3,
                 from ../../../../ext/ddtrace_profiling_native_extension/collectors_cpu_and_wall_time.c:4:
/usr/local/bundle/gems/libdatadog-0.8.0.1.0-x86_64-linux/vendor/libdatadog-0.8.0/x86_64-linux/libdatadog-x86_64-unknown-linux-gnu/lib/pkgconfig/../../include/datadog/common.h:24:52: error: missing binary operator before token "("
 #elif (defined(__has_attribute) && (__has_attribute(warn_unused_result))) || \
                                                    ^
Makefile:237: recipe for target 'collectors_cpu_and_wall_time.o' failed
make: *** [collectors_cpu_and_wall_time.o] Error 1

Looking through the gcc docs this issue seems to be documented in https://gcc.gnu.org/onlinedocs/cpp/_005f_005fhas_005fattribute.html and we seem to be using defined(__has_atribute) incorrectly.

This issue seems to be introduced in #24

To Reproduce

Steps to reproduce the behavior:

Use the ivoanjo/docker-library:ddtrace_rb_2_2_10 docker image to get a suitable old version of gcc:

$ docker run --network=host -ti -v `pwd`:/working ivoanjo/docker-library:ddtrace_rb_2_2_10 /bin/bash
root@docker-desktop:/app# gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.9/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.9.2-10+deb8u1' --with-bugurl=file:///usr/share/doc/gcc-4.9/README.Bugs --enable-languages=c,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.9 --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.9 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --with-system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.9-amd64/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.9-amd64 --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.9-amd64 --with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --enable-objc-gc --enable-multiarch --with-arch-32=i586 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.9.2 (Debian 4.9.2-10+deb8u1)

and try to compile anything that includes that header.

Expected behavior

Compiling libdatadog on this gcc version should ideally still be possible.

Errors or warnings received

(See above)

How does libdatadog help you?

There's quite a lot of code on the Ruby profiler that I'll be able to delete soon ;)

[ivoanjo]

Fixed by #48