gnu: Mark a few packages as unsupported on MIPS.

* gnu/packages/gprolog.scm (gprolog): Add 'supported-systems' field.
* gnu/packages/qemu.scm (qemu-headless): Likewise.
* gnu/packages/qt.scm (qt): Likewise.
* gnu/packages/scheme.scm (racket): Likewise.

gnu/packages/gprolog.scm

@@ -52,4 +52,7 @@
 solving over finite domains.  It accepts Prolog+ constraint programs and
 produces a compiled, native binary which can function in a stand-alone
 manner.  It also features an interactive interpreter.")
-    (license (list gpl2+ lgpl3+))))
+    (license (list gpl2+ lgpl3+))
+
+    ;; See 'configure' for the list of supported architectures.
+    (supported-systems (delete "mips64el-linux" %supported-systems))))

gnu/packages/qemu.scm

@@ -125,7 +125,10 @@ the KVM kernel module in Linux.  When using KVM, QEMU can virtualize x86,
 server and embedded PowerPC, and S390 guests.")
 
     ;; Many files are GPLv2+, but some are GPLv2-only---e.g., `memory.c'.
-    (license gpl2)))
+    (license gpl2)
+
+    ;; Several tests fail on MIPS; see <http://hydra.gnu.org/build/117914>.
+    (supported-systems (delete "mips64el-linux" %supported-systems))))
 
 (define-public qemu
   ;; QEMU with GUI support.

gnu/packages/qt.scm

@@ -145,7 +145,12 @@ X11 (yet).")
     (synopsis "Cross-platform GUI library")
     (description "Qt is a cross-platform application and UI framework for
 developers using C++ or QML, a CSS & JavaScript like language.")
-    (license lgpl2.1)))
+    (license lgpl2.1)
+
+    ;; Qt 4: 'QBasicAtomicPointer' leads to build failures on MIPS;
+    ;; see <http://hydra.gnu.org/build/112828>.
+    ;; Qt 5: assembler error; see <http://hydra.gnu.org/build/112526>.
+    (supported-systems (delete "mips64el-linux" %supported-systems))))
 
 (define-public qt-4
   (package (inherit qt)

gnu/packages/scheme.scm

@@ -422,4 +422,8 @@ implementation techniques and as an expository tool.")
 R6RS) and related languages, such as Typed Racket.  It features a compiler and
 a virtual machine with just-in-time native compilation, as well as a large set
 of libraries.")
-    (license lgpl2.0+)))
+    (license lgpl2.0+)
+
+    ;; Fails to build on MIPS with "address or size is not OS PAGE ALIGNED".
+    ;; See <http://hydra.gnu.org/build/121775>.
+    (supported-systems (delete "mips64el-linux" %supported-systems))))