To add -I/opt/include and -L/opt/lib, use --with-opt-dir=/opt
Do not override CFLAGS. If CFLAGS is provided, it is used as a replacement for default flags (i.e., '-ggdb3 -O3' will not be used unless you specify them)
Use MAINLIBS to add libraries to the ruby binary only (i.e. no c-extensions). This is ideal for allocators:
make ruby MAINLIBS=-ltcmalloc
make ruby-jemalloc RUBY_INSTALL_NAME=ruby-jemalloc MAINLIBS=-ljemalloc
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