T: isolate generated .h files when using make

When using the make system to generate T files, if you change
T_messages.txt you may have problems compiling the gnb with ninja.

If you want to see the problem:
First run: cd cmake_targets; ./build_oai --ninja --gNB
Then: cd common/utils/T
edit T_messages.txt, add a trace, for example:
---
ID = ENB_PHY_UL_TOCK
    DESC = eNodeB uplink tick - one tick per ms at start of uplink processing
    GROUP = ALL:PHY:GRAPHIC:ENB
    FORMAT = int,eNB_ID : int,frame : int,subframe
---
then run: make
then: cd cmake_targets/ran_build/build; ninja nr-softmodem
You should have an error looking like this:
---
[3/81] Checking validity of VCD files
FAILED: common/utils/T/CMakeFiles/check_vcd /tmp/develop/cmake_targets/ran_build/build/common/utils/T/CMakeFiles/check_vcd
cd /tmp/develop/cmake_targets/ran_build/build/common/utils/T && /tmp/develop/cmake_targets/ran_build/build/common/utils/T/_check_vcd /tmp/develop/common/utils/T/T_messages.txt /tmp/develop/common/utils/T/../LOG/vcd_signal_dumper.h
error: VCD_FIRST_VARIABLE is not correct in T_defs.h

You probably added a VCD trace (variable or function) but you did not
update T_messages.txt and/or T_defs.h in common/utils/T/
[...]
---

Let's first isolate the generated T .h files when using make because
we have two versions of T_IDs.h and T_messages.txt.h when mixing
make/cmake.

A next commit will regenerate T_IDs.h and T_messages.txt.h when
T_messages.txt chages for the cmake system, because this commit
does not solve the problem.

common/utils/T/tracer/Makefile

@@ -1,5 +1,5 @@
 CC=gcc
-CFLAGS=-Wall -g -pthread -DT_TRACER -I.
+CFLAGS=-Wall -g -pthread -DT_TRACER -I. -I../incgen
 
 #CFLAGS += -O3 -ffast-math -fomit-frame-pointer
 
@@ -66,9 +66,9 @@ gnb: utils.o gnb.o database.o event.o handler.o configuration.o \
          filter/filter.a
 	$(CC) $(CFLAGS) -o gnb $^ $(LIBS) $(XLIBS)
 
-multi.o: ../T_IDs.h
+multi.o: ../incgen/T_IDs.h
 
-../T_IDs.h:
+../incgen/T_IDs.h: ../T_messages.txt
 	cd .. && $(MAKE)
 
 .PHONY: all gui/gui.a view/view.a logger/logger.a filter/filter.a