Fix GNU make extension complaint in man/Makefile.am

Currently the following complaint is in the build logs

man/Makefile.am:56: warning: ':='-style assignments are not portable
man/Makefile.am:56: warning: shell for fil in $(TXT3: non-POSIX variable name
man/Makefile.am:56: (probably a GNU make extension)

Correct issue by using

configure.ac:
AC_SUBST([DOLLAR_SIGN],[$])

man/Makefile.am:
to substiture for $ as in $(shell ...
and replace := with =

configure.ac

@@ -474,6 +474,7 @@ fi
 
 AC_SUBST(LIBCOAP_NAME_SUFFIX)
 AC_SUBST(LIBCOAP_DTLS_LIB_EXTENSION_NAME)
+AC_SUBST([DOLLAR_SIGN],[$])
 
 # configure options
 # __tests__

man/Makefile.am

@@ -53,7 +53,7 @@ man7_MANS = $(MAN7)
 	$(A2X) --doctype manpage --format xhtml $<
 
 # Man pages built byt a2x based on the NAMES section of the .txt file
-A2X_EXTRA_PAGES := $(shell for fil in $(TXT3); do sed -ne '/^NAME/,/^SYNOPSIS/p;/^SYNOPSIS/q' $${fil} | \
+A2X_EXTRA_PAGES = @DOLLAR_SIGN@(shell for fil in $(TXT3) ; do sed -ne '/^NAME/,/^SYNOPSIS/p;/^SYNOPSIS/q' $${fil} | \
 	sed -ne '/coap_/{ s/ *, */\n/g; p }' | sed -ne 's/^\(coap_[a-zA-Z_]\+\).*$$/\1.3/p' ; done)
 
 # Extra man pages that need to be installed due to limit of 10