Makefile: DEBUG_TOOLS -> DEBUG and document

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

README.md

@@ -36,6 +36,7 @@ This command takes into account several environment variables:
   * `WITH_BASHCOMPLETION`  default: [auto-detect]
   * `WITH_WGQUICK`         default: [auto-detect]
   * `WITH_SYSTEMDUNITS`    default: [auto-detect]
+  * `DEBUG`                default:
 
 The first section is rather standard. The second section is not:
 
@@ -61,6 +62,8 @@ The first section is rather standard. The second section is not:
     should set it to `no`. If systemd isn't auto-detected, but you still would
     like to install it, set this to `yes`.
 
+  * `DEBUG` decides whether to build with `-g`, when set to `yes`.
+
 If you're a simple `make && make install` kind of user, you can get away with
 not setting these variables and relying on the auto-detection. However, if
 you're writing a package for a distro, you'll want to explicitly set these,

src/Makefile

@@ -43,7 +43,7 @@ CFLAGS += -std=gnu99 -D_GNU_SOURCE
 CFLAGS += -Wall -Wextra
 CFLAGS += -MMD -MP
 CFLAGS += -DRUNSTATEDIR="\"$(RUNSTATEDIR)\""
-ifeq ($(DEBUG_TOOLS),y)
+ifeq ($(DEBUG),yes)
 CFLAGS += -g
 endif
 ifeq ($(PLATFORM),linux)