build: add support for EXPORT_UNPREFIXED=no

In an EXPORT_UNPREFIXED=no build, getcontext/makecontext/setcontext/swapcontext
symbols are not provided.

Makefile

@@ -6,6 +6,11 @@ endif
 LIBDIR := /lib
 CFLAGS := -ggdb3 -O2 -Wall
 CPPFLAGS := -Iarch/${ARCH} -Iarch/common
+EXPORT_UNPREFIXED := yes
+
+ifeq ($(EXPORT_UNPREFIXED),yes)
+	CFLAGS += -DEXPORT_UNPREFIXED
+endif
 
 LIBUCONTEXT_C_SRC = $(wildcard arch/${ARCH}/*.c)
 LIBUCONTEXT_S_SRC = $(wildcard arch/${ARCH}/*.S)

arch/common/common-defs.h

@@ -36,9 +36,13 @@ __proc:							\
 	.size	__proc,.-__proc;
 #endif
 
+#ifdef EXPORT_UNPREFIXED
 #define ALIAS(__alias, __real)				\
 	.weak	__alias;				\
 	__alias = __real;
+#else
+#define ALIAS(...)
+#endif
 
 #define REG_OFFSET(__reg)       (MCONTEXT_GREGS + ((__reg) * REG_SZ))