gnu: mesa: Fix cross-compilation.

* gnu/packages/gl.scm(mesa)[arguments]<#:phases>: When cross-compiling, force
the use of the cmake method to find llvm.

Change-Id: I27f6f4d9d71028c15588eeade48d3a918c042b9c
Signed-off-by: Mathieu Othacehe <othacehe@gnu.org>

gnu/packages/gl.scm

@@ -421,6 +421,9 @@ svga,swrast,virgl,zink")))
                      ;; When cross compiling, we use cmake to find llvm, not
                      ;; llvm-config, because llvm-config cannot be executed
                      ;; see https://github.com/llvm/llvm-project/issues/58984
+                     (substitute* "meson.build"
+                       (("method : host_machine\\.system.*")
+                        "method : 'cmake',\n"))
                      (setenv "CMAKE"
                              (search-input-file
                               native-inputs "/bin/cmake")))))