THRIFT-3288 c_glib: Unit tests: Enable warnings, improve automake-variable use

This change

- Enables the same comprehensive set of warnings ("-Wall -Wextra
  -pedantic") specified for the unit tests for C++ and

- Changes the use of automake variables (e.g. AM_CPPFLAGS) to more
  closely match their intended use, as specified in the automake
  documentation.
diff --git a/lib/c_glib/test/Makefile.am b/lib/c_glib/test/Makefile.am
index 2a38878..0de7d2d 100755
--- a/lib/c_glib/test/Makefile.am
+++ b/lib/c_glib/test/Makefile.am
@@ -32,12 +32,12 @@
         gen-c_glib/t_test_thrift_test.h \
         gen-c_glib/t_test_thrift_test_types.h
 
-AM_CPPFLAGS = -g -Wall -I../src $(GLIB_CFLAGS)
+AM_CPPFLAGS = -I../src
+AM_CFLAGS = -g -Wall -Wextra -pedantic $(GLIB_CFLAGS) $(GOBJECT_CFLAGS) \
+	@GCOV_CFLAGS@
+AM_CXXFLAGS = $(AM_CFLAGS)
 AM_LDFLAGS = $(GLIB_LIBS) $(GOBJECT_LIBS) @GCOV_LDFLAGS@
 
-CFLAGS = @GCOV_CFLAGS@
-CXXFLAGS = -g
-
 check_PROGRAMS = \
   testapplicationexception \
   testtransportsocket \