CCBC-1596: Fix build with clang when lld cannot report libm location

Change-Id: I10923a2353b5ce81fad4ae89fff7d09463ba08fd
Reviewed-on: https://review.couchbase.org/c/libcouchbase/+/190111
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Brett Lawson <brett19@gmail.com>

Author: avsej

tests/CMakeLists.txt

@@ -8,9 +8,11 @@ ELSEIF (CMAKE_SYSTEM_NAME STREQUAL "Linux")
             OUTPUT_VARIABLE LIBM_SO_TRACE
             ERROR_QUIET
             OUTPUT_STRIP_TRAILING_WHITESPACE)
-    STRING(REGEX MATCH [-_a-z0-9/]+libm.so[.0-9]+ LIBM_SO_PATH ${LIBM_SO_TRACE})
-    IF (EXISTS "${LIBM_SO_PATH}")
-        GET_FILENAME_COMPONENT(CMAKE_TEST_SHARED_OBJECT ${LIBM_SO_PATH} NAME)
+    IF (LIBM_SO_TRACE)
+        STRING(REGEX MATCH [-_a-z0-9/]+libm.so[.0-9]+ LIBM_SO_PATH ${LIBM_SO_TRACE})
+        IF (EXISTS "${LIBM_SO_PATH}")
+            GET_FILENAME_COMPONENT(CMAKE_TEST_SHARED_OBJECT ${LIBM_SO_PATH} NAME)
+        ENDIF()
     ENDIF()
 ENDIF()