Avoid erroring on C++17 extensions under Clang

google/googletest@5a37b517ad introduced a check on `__has_cpp_attribute(maybe_unused)`. Under Clang it evaluates to true even in `-std=c++14` mode, but subsequently the use of the `[[maybe_unused]]` attribute triggers a `-Wc++17-extensions` warning which we elevate to an error via `-pedantic-errors`.

This was observed in Termux and in the macOS Travis CI environment, where we use Apple Clang: https://app.travis-ci.com/github/tanzislam/cryptopals/jobs/625697125

Interestingly, attempting to downgrade that error back to a warning with `-Wno-error=c++17-extensions` did not work.

A similar Clang issue with a different C++17 attribute was also observed in nlohmann/json#1535.

common_settings.mk

@@ -25,6 +25,8 @@ this_plugin := $(lastword $(MAKEFILE_LIST))
 CXX = g++
 CPP_STANDARD = -std=c++14 -pthread
 CPP_QUALITY_CHECKS = -pedantic -pedantic-errors -Wall -Wextra
+CPP_QUALITY_CHECKS += $(if $(filter clang,$(shell g++ --version)), \
+                           -Wno-c++17-extensions)
 CPP_OPTIMIZATIONS = -O3 -ffunction-sections -fdata-sections
 CPPFLAGS = $(CPP_STANDARD) $(CPP_QUALITY_CHECKS) $(CPP_OPTIMIZATIONS)
 CXXFLAGS := -isystem $(BOOST_DIR) \