jemalloc can't build when CFLAGS/CXXFLAGS contain -I options pointing to a prefix with an existing install of jemalloc

[adamnovak]

If you install one version of jemalloc into a prefix, and set your CFLAGS and CXXFLAGS environment variables to have -I /path/to/that/prefix/include in them, and then you try to build a different version of jemalloc, the build can fail.

jemalloc's build system adds -I flags pointing to the currently-being-built jemalloc's include directory to the command line after preexisting CFLAGS/CXXFLAGS flags. This means the prefix-installed jemalloc's include files will be selected over the ones actually included in the jemalloc you are trying to build, and the build will fail whenever an internal jemalloc header file differs sufficiently from the corresponding one used by the installed jemalloc.