Use standard make variable names

[mikebentley15]

There are standard make variable names. Many projects adhere to them. They are used by the internal rules defined within GNU make, but I prefer not to depend on those since it is not very clear to many how those work, including me. I'd prefer to be explicit about what the makefile is supposed to do. However, we can still adhere to the convention of variable names, so that they can be overridden or appended in an expected way by the user.

See https://www.gnu.org/software/make/manual/html_node/Implicit-Variables.html

Variables of interest:

• CXX
• CXXFLAGS
• LDFLAGS
• LDLIBS

Another common variable to be overridden is the PREFIX variable when doing installations.