Makefile: Conditional compiler and logging added

- Updated CC assignment to be conditional using ifeq
- Added logging for the compiler in use with $(info CC)

The update ensures that the compiler is set only if not explicitly customized,
promoting better portability across different environments. The additional
logging provides clarity about which compiler is in use during builds, helpful
for debugging and verification. 🛠️🖥️

Makefile

@@ -1,4 +1,6 @@
-CC         ?= gcc
+ifeq ($(origin CC),default)
+  CC := gcc
+endif
 CFLAGS     ?= -std=c11 -Wall -Wextra -O3 -MMD -MP
 PREFIX     ?= /usr/local
 SRC         = $(filter-out $(wildcard test_*.c),$(wildcard *.c))
@@ -62,6 +64,7 @@ CFLAGS += -DATHR_OS_UNIX=$(ATHR_OS_UNIX)
 CFLAGS += -DATHR_OS=$(ATHR_OS)
 
 $(info OS               = $(OS))
+$(info CC               = $(CC))
 $(info PKG_CONFIG_FOUND = $(PKG_CONFIG_FOUND))
 $(info CURSES_FOUND     = $(CURSES_FOUND))
 $(info CURSES_LIBS      = $(CURSES_LIBS))