configure: Fix googletest depedence

b4b3247afc020843ebea278bf3c4e1cbebc54968 accidentally added a hard
dependency on googletest. The behavior of PKG_CHECK_MODULES is to fail
if the package is not found if no [action-if-not-found] is provided.
This change provides a no-op [action-if-not-found] so that autodetection
does not fail if not required.

Tested:
    Ran through the unit test suite. Ran through the openbmc build
    process without the googletest dependency to make sure it builds
    now.

Change-Id: I81492833cc88b269a6d9ac1cb55b95da1975d40b
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/configure.ac b/configure.ac
index 753d04d..217e200 100644
--- a/configure.ac
+++ b/configure.ac
@@ -79,7 +79,7 @@
 
 # Check/set gtest specific functions.
 AS_IF([test "x$enable_tests" != "xno"], [
-    PKG_CHECK_MODULES([GTEST], [gtest])
+    PKG_CHECK_MODULES([GTEST], [gtest], [], [true])
 
     AX_SAVE_FLAGS_WITH_PREFIX(OLD, [CPPFLAGS])
     AX_APPEND_COMPILE_FLAGS([$GTEST_CFLAGS], [CPPFLAGS])