It turns out CMake requires the format to be:

^msgmerge \(.*\) (gettext-version-number-here)

So, this fixes CMake's test suite, and properly sets GETTEXT_VERSION_STRING
to be the same as the m4 scripts shipped.

--- gettext-tiny-0.3.0_pre1/src/msgmerge.c.old	2018-06-20 23:47:19.927932643 -0500
+++ gettext-tiny-0.3.0_pre1/src/msgmerge.c	2018-06-20 23:55:12.997464272 -0500
@@ -18,7 +18,7 @@
 __attribute__((noreturn))
 static void version(void) {
 	fprintf(stdout,
-		"these are not (GNU gettext-tools) 99.9999.9999\n");
+		"msgmerge (GNU gettext-tools compatible) 99.9999.9999\n");
 	exit(0);
 }