updated documentation to include flags for debugging

Author: avinab-neogy

docs/tutorials/building_r.md

@@ -55,7 +55,19 @@ mkdir -p $BUILDDIR
 cd $BUILDDIR
 ```
 
-**5) Configure the build**
+**5) Set CFLAGS (Optional—For Debugging C Code)**
+
+- **This step is optional and recommended for those who want to debug C code.**
+- Set the `CFLAGS` environment variable before running configure:
+
+```bash
+CFLAGS="-g -O0"
+```
+
+- After running this command, the CFLAGS setting will apply only to the
+current terminal session.
+
+**6) Configure the build**
 
 - After we change directory, we must run the configure script from the source
 directory.  This step takes ~1 minute on the codespace.
@@ -80,7 +92,7 @@ $TOP_SRCDIR/configure --with-valgrind-instrumentation=1
 
     ![alt text](../assets/rdev7.png)
 
-**6) Build R**
+**7) Build R**
 
 Having configured R, we run `make` to build R. This take 5-10 minutes on the
 codespace.
@@ -89,7 +101,7 @@ codespace.
 make
 ```
 
-**7) Check R**
+**8) Check R**
 
 Check that the build of R passes R's standard checks:
 
@@ -101,7 +113,7 @@ This takes a couple of minutes in the codespace. The check will stop with a
 error message if any of the tests fail. If this happens, see [SVN
 Help](./svn_help.md) for how to revert to a version that passes check.
 
-**8) Make R terminals use the built R**
+**9) Make R terminals use the built R**
 
 Run the `which_r` script to set which R to use for R terminals in VSCode. When
 prompted, enter the number corresponding to `r-devel`
@@ -125,7 +137,7 @@ built![^1]
 selected version is saved in the VSCode settings, so will be saved when you stop
 and restart the codespace.
 
-**9) Make contributions**
+**10) Make contributions**
 
 - After having built the current development version of R, we can now make
   changes to the source code and contribute to the project.