Make configure show it's looking for "pixman", not "xrdp_pixman"

The pixman library in not part of xrdp. If users see that "xrdp_pixman"
is not found, they might assume it is some xrdp specific library.

The Automake conditional is still XRDP_PIXMAN, but the pkg-config
variables become PIXMAN_LIBS and PIXMAN_CFLAGS.

configure.ac

@@ -201,7 +201,7 @@ then
     [AC_MSG_ERROR([please install libmp3lame-dev or lamemp3-devel])])
 fi
 
-AS_IF( [test "x$enable_pixman" = "xyes"] , [PKG_CHECK_MODULES(XRDP_PIXMAN, pixman-1 >= 0.1.0)] )
+AS_IF( [test "x$enable_pixman" = "xyes"] , [PKG_CHECK_MODULES(PIXMAN, pixman-1 >= 0.1.0)] )
 
 # checking for TurboJPEG
 if test "x$enable_tjpeg" = "xyes"

xrdp/Makefile.am

@@ -24,8 +24,8 @@ endif
 
 if XRDP_PIXMAN
 AM_CPPFLAGS += -DXRDP_PIXMAN
-AM_CPPFLAGS += $(XRDP_PIXMAN_CFLAGS)
-XRDP_EXTRA_LIBS += $(XRDP_PIXMAN_LIBS)
+AM_CPPFLAGS += $(PIXMAN_CFLAGS)
+XRDP_EXTRA_LIBS += $(PIXMAN_LIBS)
 endif
 
 sbin_PROGRAMS = \