Move to POSIX 2008 in anticipation of signals change

Technically, the signals require POSIX 2016, but the POSIX 2016 standard
requires the POSIX 2008 defines to be used, so we will use them.
diff --git a/Makefile.in b/Makefile.in
index 27f2f31..64e03f4 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -145,7 +145,7 @@
 CPPFLAGS1 = -D$(BC_ENABLED_NAME)=$(BC_ENABLED) -D$(DC_ENABLED_NAME)=$(DC_ENABLED)
 CPPFLAGS2 = $(CPPFLAGS1) -I./include/ -DVERSION=$(VERSION) %%LONG_BIT_DEFINE%%
 CPPFLAGS3 = $(CPPFLAGS2) -DEXECPREFIX=$(EXEC_PREFIX) -DMAINEXEC=$(MAIN_EXEC)
-CPPFLAGS4 = $(CPPFLAGS3) -D_POSIX_C_SOURCE=200112L -D_XOPEN_SOURCE=600
+CPPFLAGS4 = $(CPPFLAGS3) -D_POSIX_C_SOURCE=200809L -D_XOPEN_SOURCE=700
 CPPFLAGS5 = $(CPPFLAGS4) -DBC_NUM_KARATSUBA_LEN=$(BC_NUM_KARATSUBA_LEN)
 CPPFLAGS6 = $(CPPFLAGS5) -DBC_ENABLE_NLS=$(BC_ENABLE_NLS) -DBC_ENABLE_PROMPT=$(BC_ENABLE_PROMPT)
 CPPFLAGS7 = $(CPPFLAGS6) -D$(BC_ENABLE_EXTRA_MATH_NAME)=$(BC_ENABLE_EXTRA_MATH)