Some Makefile variables are reserved by the GNU Coding Standards
for the use of the “user”—the person building the package. For
instance, CFLAGS is one such variable.
Sometimes package developers are tempted to set user variables such as
CFLAGS because it appears to make their job easier. However,
the package itself should never set a user variable, particularly not
to include switches that are required for proper compilation of the
package. Since these variables are documented as being for the
package builder, that person rightfully expects to be able to override
any of these variables at build time.
To get around this problem, Automake introduces an automake-specific
shadow variable for each user flag variable. (Shadow variables are
not introduced for variables like CC, where they would make no
sense.) The shadow variable is named by prepending ‘AM_’ to the
user variable’s name. For instance, the shadow variable for
YFLAGS is AM_YFLAGS. The package maintainer—that is,
the author(s) of the Makefile.am and configure.ac
files—may adjust these shadow variables however necessary.
See Flag Variables Ordering, for more discussion about these variables and how they interact with per-target variables.