Include flags from environment variables set externally

[jakirkham]

In build systems (like conda-forge), some flags like CXXFLAGS are set as part of the build process to provide consistency across package builds and point to standard locations that may be part of the build system

IIUC it looks like scalene sets its own CXXFLAGS and doesn't pick up these values from the environment

scalene/GNUmakefile

Line 10 in 0ac9a1c

CXXFLAGS = -std=c++14 -Wall -g -O3 -DNDEBUG -D_REENTRANT=1 -DHL_USE_XXREALLOC=1 -pipe -fno-builtin-malloc -fvisibility=hidden -Wno-unused-result

scalene/Makefile

Line 6 in 0ac9a1c

CXXFLAGS = /Ox /DNDEBUG /std:c++14 /Zi

Would it be possible for scalene to inherit environment variables set outside make and prepend/append to them?

[emeryberger]

Would this solve the Python 3.8 build problem? If so - and if it doesn't interfere with regular builds with PyPi - we'd welcome a PR.