THRIFT-3285 c_glib: Library: Build with all warnings enabled

This matches the settings ("-Wall -Wextra -pedantic") used to build
the C++ library.
diff --git a/lib/c_glib/Makefile.am b/lib/c_glib/Makefile.am
index 67ebf20..58e7329 100755
--- a/lib/c_glib/Makefile.am
+++ b/lib/c_glib/Makefile.am
@@ -25,6 +25,7 @@
 pkgconfig_DATA = thrift_c_glib.pc
 
 AM_CPPFLAGS = -Isrc -I src/thrift/c_glib
+AM_CFLAGS = -Wall -Wextra -pedantic
 
 # Define the source files for the module