Merge #1203: Do not link bench and ctime_tests to COMMON_LIB

ef39721 Do not link `bench` and `ctime_tests` to `COMMON_LIB` (Hennadii Stepanov)

Pull request description:

  The `bench` and `ctime_tests` binaries are users of the library, they should only be linked to the library, not the objects it was built from.

ACKs for top commit:
  sipa:
    utACK ef39721
  real-or-random:
    utACK ef39721

Tree-SHA512: 8bf8330adcce9bf6b21aceacf86e6aff7594762ab68b09257cfe2904fa0ce827377d5a13c0bed5acde74a2b420bb49460657c66d0068ecbe36dc162140876be4

Author: real-or-random

Makefile.am

@@ -103,7 +103,7 @@ noinst_PROGRAMS =
 if USE_BENCHMARK
 noinst_PROGRAMS += bench bench_internal bench_ecmult
 bench_SOURCES = src/bench.c
-bench_LDADD = libsecp256k1.la $(COMMON_LIB)
+bench_LDADD = libsecp256k1.la
 bench_CPPFLAGS = $(SECP_CONFIG_DEFINES)
 bench_internal_SOURCES = src/bench_internal.c
 bench_internal_LDADD = $(COMMON_LIB) $(PRECOMPUTED_LIB)
@@ -134,7 +134,7 @@ endif
 if USE_CTIME_TESTS
 noinst_PROGRAMS += ctime_tests
 ctime_tests_SOURCES = src/ctime_tests.c
-ctime_tests_LDADD = libsecp256k1.la $(COMMON_LIB)
+ctime_tests_LDADD = libsecp256k1.la
 ctime_tests_CPPFLAGS = $(SECP_CONFIG_DEFINES)
 endif