Allow usage when fork() is not available

When fork() is not available, we need to define NO_ZPIPE so that
libXpm doesn't try to fork/exec to use a pipe to uncompress compressed
.xpm files. There is obviously a loss of functionality, but loading
uncompressed .xpm files should continue to work.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/configure.ac
===================================================================
--- a/configure.ac
+++ b/configure.ac
@@ -22,6 +22,7 @@
 
 # Checks for library functions
 AC_CHECK_FUNCS([strlcat])
+AC_CHECK_FUNC([fork],[], AC_DEFINE(NO_ZPIPE))
 
 # Obtain compiler/linker options for dependencies
 PKG_CHECK_MODULES(XPM, xproto x11)