Add support for "--enable-code-coverage" option

This change adds support for a new option that can be passed to the
configure script: "--enable-code-coverage". Further, the "--enable-gcov"
option has been removed, as this new option provides the same
functionality (plus more).

When using this new option the following make targets are available:

 * check-code-coverage
 * code-coverage-capture
 * code-coverage-clean

Note: these make targets can only be run from the root of the project.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Prakash Surya <prakash.surya@delphix.com>
Closes #6670

config/Rules.am

@@ -6,6 +6,7 @@ AM_CFLAGS += ${NO_UNUSED_BUT_SET_VARIABLE}
 AM_CFLAGS += ${NO_BOOL_COMPARE}
 AM_CFLAGS += -fno-strict-aliasing
 AM_CFLAGS += -std=gnu99
+AM_CFLAGS += $(CODE_COVERAGE_CFLAGS)
 AM_CPPFLAGS  = -D_GNU_SOURCE -D__EXTENSIONS__ -D_REENTRANT
 AM_CPPFLAGS += -D_POSIX_PTHREAD_SEMANTICS -D_FILE_OFFSET_BITS=64
 AM_CPPFLAGS += -D_LARGEFILE64_SOURCE -DHAVE_LARGE_STACKS=1
@@ -14,3 +15,4 @@ AM_CPPFLAGS += -DLIBEXECDIR=\"$(libexecdir)\"
 AM_CPPFLAGS += -DRUNSTATEDIR=\"$(runstatedir)\"
 AM_CPPFLAGS += -DSBINDIR=\"$(sbindir)\"
 AM_CPPFLAGS += -DSYSCONFDIR=\"$(sysconfdir)\"
+AM_CPPFLAGS += $(CODE_COVERAGE_CPPFLAGS)