configure: properly check if -lrt and -lm is needed

Fixes build against uClibc.

uClibc provides 2 versions of clock_gettime(), one with realtime
support and one without (this is so you can avoid linking in -lrt
unless actually needed). This means that the clock_gettime() don't
need -lrt. We still need it for timer_create() so we check for this
function in addition.

We also need check if -lm is needed for isnan().

Both -lm and -lrt are needed for libs_qga.
Signed-off-by: NNatanael Copa <ncopa@alpinelinux.org>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

configure

@@ -2670,18 +2670,45 @@ EOF
 fi
 
 
+##########################################
+# Do we need libm
+cat > $TMPC << EOF
+#include <math.h>
+int main(void) { return isnan(sin(0.0)); }
+EOF
+if compile_prog "" "" ; then
+  :
+elif compile_prog "" "-lm" ; then
+  LIBS="-lm $LIBS"
+  libs_qga="-lm $libs_qga"
+else
+  echo
+  echo "Error: libm check failed"
+  echo
+  exit 1
+fi
+
 ##########################################
 # Do we need librt
+# uClibc provides 2 versions of clock_gettime(), one with realtime
+# support and one without. This means that the clock_gettime() don't
+# need -lrt. We still need it for timer_create() so we check for this
+# function in addition.
 cat > $TMPC <<EOF
 #include <signal.h>
 #include <time.h>
-int main(void) { return clock_gettime(CLOCK_REALTIME, NULL); }
+int main(void) {
+  timer_create(CLOCK_REALTIME, NULL, NULL);
+  return clock_gettime(CLOCK_REALTIME, NULL);
+}
 EOF
 
 if compile_prog "" "" ; then
   :
-elif compile_prog "" "-lrt" ; then
+# we need pthread for static linking. use previous pthread test result
+elif compile_prog "" "-lrt $pthread_lib" ; then
   LIBS="-lrt $LIBS"
+  libs_qga="-lrt $libs_qga"
 fi
 
 if test "$darwin" != "yes" -a "$mingw32" != "yes" -a "$solaris" != yes -a \