gnutls: build the test targets with -lgnu as they need memmem(3).

* Without the "-lgnu" we would get an undefined reference to `memmem'
  in function hsk_hook_cb (resume_x509-resume.o from tests/resume.c)
* Define TEST_REQUIRES instead of TEST_PREREQUIRES, and drop cmd:cvs
  from it, as cvs is currently failing to build on x86_64.
* No need to revbump as the changes only affect TEST().

net-libs/gnutls/gnutls35-3.5.18.recipe

@@ -130,8 +130,7 @@ BUILD_PREREQUIRES="
 	cmd:pkg_config$secondaryArchSuffix
 	"
 
-TEST_PREREQUIRES="
-	cmd:cvs
+TEST_REQUIRES="
 	cmd:git
 	"
 
@@ -169,5 +168,5 @@ INSTALL()
 
 TEST()
 {
-	make check
+	make check LIBS="-lgnu"
 }

net-libs/gnutls/gnutls36-3.6.2.recipe

@@ -129,8 +129,7 @@ BUILD_PREREQUIRES="
 	cmd:pkg_config$secondaryArchSuffix
 	"
 
-TEST_PREREQUIRES="
-	cmd:cvs
+TEST_REQUIRES="
 	cmd:git
 	"
 
@@ -168,5 +167,5 @@ INSTALL()
 
 TEST()
 {
-	make check
+	make check LIBS="-lgnu"
 }