contrib: let Meson find CMake packages

libplacebo should be able to find glslang that way since it doesn't export a
pkg-config file.

contrib/src/gen-meson-crossfile.py

@@ -62,3 +62,11 @@ args.file.write("endian = 'little'\n")
 # Get first part of triplet
 cpu = os.environ.get('HOST', '').split('-')[0]
 args.file.write("cpu = '{}'\n".format(cpu))
+
+# CMake section
+args.file.write("\n[cmake]\n")
+_add_environ_val('CMAKE_C_COMPILER', 'CC')
+_add_environ_val('CMAKE_CXX_COMPILER', 'CXX')
+_add_environ_val('CMAKE_SYSTEM_NAME', 'CMAKE_SYSTEM_NAME')
+_add_environ_val('CMAKE_SYSTEM_PROCESSOR', 'ARCH')
+

contrib/src/main.mak

@@ -487,6 +487,8 @@ ifdef HAVE_CROSS_COMPILE
 MESONFLAGS += --cross-file $(abspath crossfile.meson)
 MESON = env -i PATH="$(PREFIX)/bin:$(PATH)" \
 	PKG_CONFIG_PATH="$(PKG_CONFIG_PATH)" \
+	CMAKE="$(command -v cmake)" \
+	CMAKE_PREFIX_PATH="$(PREFIX)" \
 	meson -Dpkg_config_path="$(PKG_CONFIG_PATH)" \
 	$(MESONFLAGS)