Makefile: Fix code coverage flags

We need to apply CFLAGS and CXXFLAGS to all objects being built, or else
the coverage information will not be complete and correct.

Change-Id: I73b639b2a67037ffa01bbc4b50492d8f52d7beab
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/Makefile.am b/Makefile.am
index f3a1997..db8a54e 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -26,6 +26,11 @@
 CODE_COVERAGE_GENHTML_OPTIONS += --prefix $(abs_srcdir) --prefix $(abs_builddir)
 export CODE_COVERAGE_GENHTML_OPTIONS
 
+export AM_CFLAGS = $(CODE_COVERAGE_CFLAGS)
+export AM_CXXFLAGS = $(CODE_COVERAGE_CXXFLAGS)
+
+export LDADD = $(CODE_COVERAGE_LIBS)
+
 if AUTOCONF_CODE_COVERAGE_2019_01_06
 include $(top_srcdir)/aminclude_static.am
 clean-local: code-coverage-clean