Build: Leave LIBS alone during configure

Put the extra libraries into SR_EXTRA_LIBS instead of LIBS.
Create an SR_CHECK_LIBS macro to make that easy. Substitute
SR_EXTRA_LIBS into libsigrok.pc, too.

Makefile.am

@@ -419,7 +419,7 @@ libsigrok_la_SOURCES += \
 	src/hardware/zeroplus-logic-cube/api.c
 endif
 
-libsigrok_la_LIBADD = $(LIBSIGROK_LIBS)
+libsigrok_la_LIBADD = $(SR_EXTRA_LIBS) $(LIBSIGROK_LIBS)
 libsigrok_la_LDFLAGS = -version-info $(SR_LIB_VERSION) -no-undefined
 
 library_includedir = $(includedir)/libsigrok
@@ -487,7 +487,7 @@ tests_main_SOURCES = \
 	tests/device.c \
 	tests/trigger.c
 
-tests_main_LDADD = libsigrok.la $(CHECK_LIBS)
+tests_main_LDADD = libsigrok.la $(SR_EXTRA_LIBS) $(CHECK_LIBS)
 
 endif
 
@@ -501,7 +501,7 @@ lib_LTLIBRARIES += bindings/cxx/libsigrokcxx.la
 
 bindings_cxx_libsigrokcxx_la_SOURCES = bindings/cxx/classes.cpp
 
-bindings_cxx_libsigrokcxx_la_LIBADD = libsigrok.la $(LIBSIGROKCXX_LIBS)
+bindings_cxx_libsigrokcxx_la_LIBADD = libsigrok.la $(SR_EXTRA_LIBS) $(LIBSIGROKCXX_LIBS)
 bindings_cxx_libsigrokcxx_la_LDFLAGS = -version-info $(SR_LIB_VERSION) -no-undefined
 
 bindings_cxx_libsigrokcxx_la_includedir = $(includedir)/libsigrokcxx

configure.ac

@@ -122,6 +122,12 @@ AC_CHECK_HEADERS([sys/mman.h], [SR_APPEND([sr_deps_avail], [sys_mman_h])])
 AC_CHECK_HEADERS([sys/ioctl.h], [SR_APPEND([sr_deps_avail], [sys_ioctl_h])])
 AC_CHECK_HEADERS([sys/timerfd.h], [SR_APPEND([sr_deps_avail], [sys_timerfd_h])])
 
+# We need to link against the Winsock2 library for SCPI over TCP.
+AS_CASE([$host], [*-mingw*], [SR_EXTRA_LIBS='-lws2_32'], [SR_EXTRA_LIBS=])
+
+# libm (the standard math library) is always needed.
+SR_SEARCH_LIBS([SR_EXTRA_LIBS], [pow], [m])
+
 # RPC is only needed for VXI support.
 AC_CACHE_CHECK([for RPC support], [sr_cv_have_rpc],
 	[AC_LINK_IFELSE([AC_LANG_PROGRAM(
@@ -406,14 +412,6 @@ AC_DEFINE_UNQUOTED([CONF_LIBZIP_VERSION], ["$sr_libzip_version"],
 AC_DEFINE_UNQUOTED([CONF_HOST], ["$host"],
 	[The canonical host libsigrok will run on.])
 
-AC_LANG([C])
-
-# libm (the standard math library) is always needed.
-AC_SEARCH_LIBS([pow], [m])
-
-# We need to link against the Winsock2 library for SCPI over TCP.
-AS_CASE([$host], [*mingw*], [LIBS="$LIBS -lws2_32"])
-
 AC_SUBST([FIRMWARE_DIR], ['$(datadir)/sigrok-firmware'])
 AC_SUBST([MAKEFLAGS], ['--no-print-directory'])
 AC_SUBST([AM_LIBTOOLFLAGS], ['--silent'])

libsigrok.pc.in

@@ -10,5 +10,5 @@ Requires: glib-2.0
 Requires.private: @SR_PKGLIBS@
 Version: @SR_PACKAGE_VERSION@
 Libs: -L${libdir} -lsigrok
-Libs.private: -lm
+Libs.private: @SR_EXTRA_LIBS@
 Cflags: -I${includedir}

m4/sigrok.m4

@@ -16,7 +16,7 @@
 ## You should have received a copy of the GNU General Public License
 ## along with sigrok. If not, see <http://www.gnu.org/licenses/>.
 
-#serial 20150823
+#serial 20150824
 
 ## SR_APPEND(var-name, [list-sep,] element)
 ##
@@ -31,6 +31,19 @@ m4_assert([$# >= 2])[]dnl
 $1=[$]{$1[}]m4_if([$#], [2], [[$]{$1:+' '}$2], [[$]{$1:+$2}$3])[]dnl
 ])
 
+## SR_PREPEND(var-name, [list-sep,] element)
+##
+## Prepend the shell word <element> to the shell variable named <var-name>,
+## suffixed by <list-sep> unless the list was empty before prepending. If
+## only two arguments are supplied, <list-sep> defaults to a single space
+## character.
+##
+AC_DEFUN([SR_PREPEND],
+[dnl
+m4_assert([$# >= 2])[]dnl
+$1=m4_if([$#], [2], [$2[$]{$1:+' '}], [$3[$]{$1:+$2}])[$]$1[]dnl
+])
+
 ## _SR_PKG_VERSION_SET(var-prefix, pkg-name, tag-prefix, base-version, major, minor, [micro])
 ##
 m4_define([_SR_PKG_VERSION_SET],
@@ -115,6 +128,24 @@ _SR_LIB_VERSION_SET([$1],
 	[$2], m4_unquote(m4_split([$2], [:])))
 ])
 
+## SR_SEARCH_LIBS(libs-var, function, search-libs,
+##                [action-if-found], [action-if-not-found], [other-libs])
+##
+## Same as AC_SEARCH_LIBS, except that the result is prepended
+## to <libs-var> instead of LIBS. Calls AC_SUBST on <libs-var>.
+##
+AC_DEFUN([SR_SEARCH_LIBS],
+[dnl
+m4_assert([$# >= 3])[]dnl
+sr_sl_save_LIBS=$LIBS
+AC_SEARCH_LIBS([$2], [$3],,, m4_join([$6], [[$]$1]))
+LIBS=$sr_sl_save_LIBS
+AS_CASE([$ac_cv_search_$2], [no*],,
+	[SR_PREPEND([$1], [$ac_cv_search_$2])])
+m4_ifvaln([$4$5], [AS_IF([test "x$ac_cv_search_$2" = xno], [$5], [$4])])[]dnl
+AC_SUBST([$1])[]dnl
+])
+
 ## _SR_VAR_SUMMARY(tag, var-name, line-leader, align-columns, align-char)
 ##
 m4_define([_SR_VAR_SUMMARY], [dnl