eclass/toolchain.eclass: Add := operators to a number of critical deps

We should annotate a number of critical library dependencies of the
compiler with := operators to ensure that the compiler gets rebuilt in
time.

TODO: We should also check for FEATURES=preserve-libs / print a warning
before attempting to upgrade a library with soname change. Otherwise,
users might be off to a rocky ride.

Closes: https://bugs.gentoo.org/642316

eclass/toolchain.eclass

@@ -166,15 +166,15 @@ RDEPEND="sys-libs/zlib
 tc_version_is_at_least 3 && RDEPEND+=" virtual/libiconv"
 
 if tc_version_is_at_least 4 ; then
-	GMP_MPFR_DEPS=">=dev-libs/gmp-4.3.2:0 >=dev-libs/mpfr-2.4.2:0"
+	GMP_MPFR_DEPS=">=dev-libs/gmp-4.3.2:0= >=dev-libs/mpfr-2.4.2:0="
 	if tc_version_is_at_least 4.3 ; then
 		RDEPEND+=" ${GMP_MPFR_DEPS}"
 	elif in_iuse fortran ; then
 		RDEPEND+=" fortran? ( ${GMP_MPFR_DEPS} )"
 	fi
 fi
 
-tc_version_is_at_least 4.5 && RDEPEND+=" >=dev-libs/mpc-0.8.1:0"
+tc_version_is_at_least 4.5 && RDEPEND+=" >=dev-libs/mpc-0.8.1:0="
 
 if in_iuse objc-gc ; then
 	if tc_version_is_at_least 7 ; then
@@ -188,8 +188,8 @@ if in_iuse graphite ; then
 	elif tc_version_is_at_least 4.8 ; then
 		RDEPEND+="
 			graphite? (
-				>=dev-libs/cloog-0.18.0
-				>=dev-libs/isl-0.11.1
+				>=dev-libs/cloog-0.18.0:0=
+				>=dev-libs/isl-0.11.1:0=
 			)"
 	fi
 fi