add support for custom static libpcre

When building, the Makefile now checks if ../pcre exists and uses it for
the libpcre include and library files.  It assumes that such a libpcre
should be linked statically with libbitconvert.  This option is useful on
systems where a binary distributable that uses libbitconvert should contain
both libbitconvert and libpcre, such as on Windows and Mac OS X.

Makefile

@@ -17,8 +17,11 @@
 
 # remove gcc-specific options in CFLAGS to use a different CC
 CC = gcc
-CFLAGS = -ansi -pedantic -Wall -Wextra -Werror
-LDFLAGS = -lpcre
+
+# if ../pcre exists, assume it contains a static libpcre and use it
+CFLAGS = -ansi -pedantic -Wall -Wextra -Werror \
+	$(shell test -d ../pcre && echo -I../pcre -DPCRE_STATIC=1)
+LDFLAGS = $(shell test -d ../pcre && echo -L../pcre) -lpcre
 
 .PHONY: all clean