unmodified environments. Flags obtained from environment variables
are not subject to the security limitations described above.

All the cgo CPPFLAGS and CFLAGS directives in a package are concatenated and
used to compile C files in that package. All the CPPFLAGS and CXXFLAGS
directives in a package are concatenated and used to compile C++ files in that
package. All the CPPFLAGS and FFLAGS directives in a package are concatenated