README.md: clarify requirements

Clarify that GTK3 and its development headers are required to build the
gzbd and gzbd-viewer applications by mentioning these in the requirement
list.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>

README.md

@@ -67,18 +67,20 @@ THAT ARE VENDOR SAMPLES OR NOT PUBLICLY AVAILABLE.
 *libzbd* requires the following packages for compilation:
 
 * autoconf
+* autoconf-archive
 * automake
 * libtool
 * m4
-* autoconf-archive
+* GTK3 and GTK3 development headers (when building the *gzbd* and *gzbd-viewer*
+  graphical applications)
 
 Since *libzbd* uses Linux(tm) kernel zoned block device interface, compilation
 must be done on a system where the kernel header file *blkzoned.h* for zoned
 block devices interface is installed under /usr/include/linux/. This implies
 that the kernel version must be higher than version 4.10.0.
 
-To automatically enable compiling the *gzbd* and *gzbd-viewer* graphical
-applications, GTK3 and GTK3 development packages must be installed.
+If the GTK3 and GTK3 development packages are not installed, compilation of the
+*gzbd* and *gzbd-viewer* graphical applications is automatically disabled.
 
 ### Compilation