cmake: Fix find_program usage for 3.15

yuzu requires CMake 3.15 yet find_program was using REQUIRED, which is
only available on 3.18 and later. Instead, we check for
"<VAR>-NOTFOUND".

In addition, check for additional requirements before building libusb or
FFmpeg with autotools. Otherwise, CMake configuration will pass yet
compilation will fail.

CMakeLists.txt

@@ -473,7 +473,15 @@ if (YUZU_USE_BUNDLED_FFMPEG)
 
         # FFmpeg has source that requires one of nasm or yasm to assemble it.
         # REQUIRED throws an error if not found here during configuration rather than during compilation.
-        find_program(ASSEMBLER NAMES nasm yasm REQUIRED)
+        find_program(ASSEMBLER NAMES nasm yasm)
+        if ("${ASSEMBLER}" STREQUAL "ASSEMBLER-NOTFOUND")
+            message(FATAL_ERROR "One of either `nasm` or `yasm` not found but is required.")
+        endif()
+
+        find_program(AUTOCONF autoconf)
+        if ("${AUTOCONF}" STREQUAL "AUTOCONF-NOTFOUND")
+            message(FATAL_ERROR "Required program `autoconf` not found.")
+        endif()
 
         set(FFmpeg_PREFIX ${PROJECT_SOURCE_DIR}/externals/ffmpeg)
         set(FFmpeg_BUILD_DIR ${PROJECT_BINARY_DIR}/externals/ffmpeg)

externals/libusb/CMakeLists.txt

@@ -5,6 +5,17 @@ if (MINGW OR (${CMAKE_SYSTEM_NAME} MATCHES "Linux"))
     # GNU toolchains for some reason doesn't work with the later half of this CMakeLists after
     # updating to 1.0.24, so we do it the old-fashioned way for now.
 
+    # Require autoconf and libtoolize here, rather than crash during compilation
+    find_program(AUTOCONF autoconf)
+    if ("${AUTOCONF}" STREQUAL "AUTOCONF-NOTFOUND")
+        message(FATAL_ERROR "Required program `autoconf` not found.")
+    endif()
+
+    find_program(LIBTOOLIZE libtoolize)
+    if ("${LIBTOOLIZE}" STREQUAL "LIBTOOLIZE-NOTFOUND")
+        message(FATAL_ERROR "Required program `libtoolize` not found.")
+    endif()
+
     set(LIBUSB_PREFIX "${CMAKE_CURRENT_BINARY_DIR}/libusb")
     set(LIBUSB_SRC_DIR "${CMAKE_CURRENT_SOURCE_DIR}/libusb")
 

src/video_core/host_shaders/CMakeLists.txt

@@ -18,7 +18,10 @@ set(SHADER_FILES
     vulkan_uint8.comp
 )
 
-find_program(GLSLANGVALIDATOR "glslangValidator" REQUIRED)
+find_program(GLSLANGVALIDATOR "glslangValidator")
+if ("${GLSLANGVALIDATOR}" STREQUAL "GLSLANGVALIDATOR-NOTFOUND")
+    message(FATAL_ERROR "Required program `glslangValidator` not found.")
+endif()
 
 set(GLSL_FLAGS "")
 set(QUIET_FLAG "--quiet")