makefile: automatic Arch Linux detection (#1332)

This commit is a port of a detection method used in koboldcpp's Makefile in order to automatically set the -lcblas option on Arch Linux

Makefile

@@ -107,7 +107,11 @@ ifndef LLAMA_NO_ACCELERATE
 endif
 ifdef LLAMA_OPENBLAS
 	CFLAGS  += -DGGML_USE_OPENBLAS -I/usr/local/include/openblas
-	LDFLAGS += -lopenblas
+	ifneq ($(shell grep -e "Arch Linux" -e "ID_LIKE=arch" /etc/os-release 2>/dev/null),)
+		LDFLAGS += -lopenblas -lcblas
+	else
+		LDFLAGS += -lopenblas
+	endif
 endif
 ifdef LLAMA_CUBLAS
 	CFLAGS    += -DGGML_USE_CUBLAS -I/usr/local/cuda/include -I/opt/cuda/include -I$(CUDA_PATH)/targets/x86_64-linux/include

README.md

@@ -216,7 +216,6 @@ Building the program with BLAS support may lead to some performance improvements
       ```bash
       make LLAMA_OPENBLAS=1
       ```
-      Note: In order to build on Arch Linux with OpenBLAS support enabled you must edit the Makefile adding at the end of the line 105: `-lcblas`
 
     - On Windows: