CXX override is ignored

[krobelus]

I'm on Ubuntu 20 LTS where /usr/bin/c++ is g++ 9.
Using that, my attempt at compiling watchman (or one of its dependencies like folly) fails due to some static assertions that require GCC 10 or higher.

After installing g++-10 and running
CC=gcc-10 CXX=g++-10 ./autogen.sh

the build failed with the same symptoms because the build system hardcodes /usr/bin/c++,
ignoring my CXX override.

I'm not sure if this is an issue in watchman or folly.

[112/351] Building CXX object CMakeFiles/folly_base.dir/folly/executors/QueuedImmediateExecutor.cpp.o
FAILED: CMakeFiles/folly_base.dir/folly/executors/QueuedImmediateExecutor.cpp.o 
/usr/bin/c++  -DBOOST_ALL_NO_LIB -DFOLLY_XLOG_STRIP_PREFIXES=\"/tmp/fbcode_builder_getdeps-ZhomeZjohannesZgitZwatchmanZbuildZfbcode_builder/repos/github.com-facebook-folly.git:/tmp/fbcode_builder_getdeps-ZhomeZjohannesZgitZwatchmanZbuildZfbcode_builder/build/folly\" -DGFLAGS_IS_A_DLL=0 -D_GNU_SOURCE -D_REENTRANT -I/tmp/fbcode_builder_getdeps-ZhomeZjohannesZgitZwatchmanZbuildZfbcode_builder/repos/github.com-facebook-folly.git -I. -I/tmp/fbcode_builder_getdeps-ZhomeZjohannesZgitZwatchmanZbuildZfbcode_builder/installed/fast_float-0xoBOutfUe7YYUA0cV7o9b81wDRdxzXG0WaCStxaKt8/include -I/tmp/fbcode_builder_getdeps-ZhomeZjohannesZgitZwatchmanZbuildZfbcode_builder/installed/glog-Z3Bv8qUgM2N4UGF5J8d6PF13OblEMrFRRky0evssbA4/include -I/tmp/fbcode_builder_getdeps-ZhomeZjohannesZgitZwatchmanZbuildZfbcode_builder/installed/xz-_MLQjX6hxWf4qxiG4eStOy_R2AOD0WBiSZtxnhAjZyA/include -I/usr/include/libdwarf -I/tmp/fbcode_builder_getdeps-ZhomeZjohannesZgitZwatchmanZbuildZfbcode_builder/installed/libunwind-5-TUqMxr9zCnBI8TNPmKJawIPF3BSBosl2IP7dB4l9k/include -I/tmp/fbcode_builder_getdeps-ZhomeZjohannesZgitZwatchmanZbuildZfbcode_builder/installed/fmt-lwERI9X-Nyh7FWmbE3WF67IHn3ra_ctPGuLdjFvQB1c/include -O2 -g -DNDEBUG   -g -finput-charset=UTF-8 -fsigned-char -Wall -Wno-deprecated -Wno-deprecated-declarations -Wno-sign-compare -Wno-unused -Wuninitialized -Wunused-label -Wunused-result -Wno-psabi -Wshadow-compatible-local -Wno-noexcept-type -faligned-new -fopenmp -std=gnu++17 -MD -MT CMakeFiles/folly_base.dir/folly/executors/QueuedImmediateExecutor.cpp.o -MF CMakeFiles/folly_base.dir/folly/executors/QueuedImmediateExecutor.cpp.o.d -o CMakeFiles/folly_base.dir/folly/executors/QueuedImmediateExecutor.cpp.o -c /tmp/fbcode_builder_getdeps-ZhomeZjohannesZgitZwatchmanZbuildZfbcode_builder/repos/github.com-facebook-folly.git/folly/executors/QueuedImmediateExecutor.cpp

[mszabo-wikia]

Passing --extra-cmake-defines '{"CMAKE_C_COMPILER":"gcc-10", "CMAKE_CXX_COMPILER":"g++-10"}' to the getdeps build call that the convenience script invokes should allow doing that.