Fix build for 64bit field under OSX #70

• caused by https://github.com/bitcoin/secp256k1/commit/8881212ebc43e67052ec06dec8beb459769fbab7
• OSX's ar tool doesn't work for empty archives ("ar: no archive members specified")
• repaired by moving entire commit under USE_ASM

Looks good to me; comments, @theuni?

Verified compile on OSX 10.10

Well, the intention for the _common lib was for it to contain anything necessary for bench/test/lib. If we're going to change its meaning to just mean the asm bits, let's be explicit about that. Also, it can be stuffed into a var to avoid adding all the extra if's. Something like:

diff --git a/Makefile.am b/Makefile.am
index a9df7d5..9207688 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -1,8 +1,16 @@
 ACLOCAL_AMFLAGS = -I m4
 INCLUDES = $(SECP_INCLUDES)
 lib_LTLIBRARIES = libsecp256k1.la
-noinst_LTLIBRARIES = libsecp256k1_common.la
-include_HEADERS = include/secp256k1.h
+noinst_LTLIBRARIES=
+
+if USE_ASM
+ASM_LIB=libsecp256k1_asm.la
+noinst_LTLIBRARIES += $(ASM_LIB)
+libsecp256k1_asm_la_SOURCES = src/field_5x52_asm.asm
+else
+ASM_LIB=
+endif
+
 noinst_HEADERS =
 noinst_HEADERS += src/group.h
 noinst_HEADERS += src/group_impl.h
@@ -33,14 +41,9 @@ noinst_HEADERS += src/field_impl.h
 pkgconfigdir = $(libdir)/pkgconfig
 pkgconfig_DATA = libsecp256k1.pc

-libsecp256k1_common_la_SOURCES =
-if USE_ASM
-libsecp256k1_common_la_SOURCES += src/field_5x52_asm.asm
-endif
-
 libsecp256k1_la_SOURCES = src/secp256k1.c
 libsecp256k1_la_CPPFLAGS = -I$(top_srcdir)/include $(SECP_INCLUDES)
-libsecp256k1_la_LIBADD = libsecp256k1_common.la $(SECP_LIBS)
+libsecp256k1_la_LIBADD = $(ASM_LIB) $(SECP_LIBS)

 noinst_PROGRAMS =
 if USE_BENCHMARK
@@ -54,7 +57,7 @@ if USE_TESTS
 noinst_PROGRAMS += tests
 tests_SOURCES = src/tests.c
 tests_CPPFLAGS = -DVERIFY $(SECP_TEST_INCLUDES)
-tests_LDADD = libsecp256k1_common.la $(SECP_LIBS) $(SECP_TEST_LIBS)
+tests_LDADD = $(ASM_LIB) $(SECP_LIBS) $(SECP_TEST_LIBS)
 tests_LDFLAGS = -static
 TESTS = tests
 endif

@theuni I don't think the definition of common was changed; it's just that it currently only contains asm stuff, and so when not using asm, it would be empty (causing OSX's ar to choke). Adding COMMON_LIB variable would be fine.

COMMON_LIB added and rebased