Fix musl gcc building biscuit under Linux

biscuit/CMakeLists.txt

@@ -9,6 +9,19 @@ aux_source_directory(c/libs LIBS)
 include_directories(c/include)
 set(EXECUTABLE_OUTPUT_PATH ${ARCH})
 
+# Find the kernel release
+execute_process(
+  COMMAND uname -r
+  OUTPUT_VARIABLE KERNEL_RELEASE
+  OUTPUT_STRIP_TRAILING_WHITESPACE
+)
+
+# Find the headers
+find_path(KERNEL_HEADERS_DIR
+  include/linux/user.h
+  PATHS /usr/lib/modules/${KERNEL_RELEASE}/build
+)
+
 # Toolchain
 if (${ARCH} STREQUAL i386)
     if(APPLE)
@@ -18,6 +31,8 @@ if (${ARCH} STREQUAL i386)
 elseif (${ARCH} STREQUAL x86_64)
     if(APPLE)
         set(PREFIX          x86_64-linux-musl-)
+    else ()
+        set(PREFIX          musl-)
     endif ()
     set(CMAKE_C_FLAGS       "-m64 -mno-red-zone")
 elseif (${ARCH} STREQUAL riscv32)
@@ -29,10 +44,10 @@ elseif (${ARCH} STREQUAL riscv64)
 elseif (${ARCH} STREQUAL aarch64)
     if(APPLE)
         set(PREFIX          aarch64-none-elf-)
-    else()
+    else ()
         set(PREFIX          aarch64-elf-)
     endif ()
-else()
+else ()
     message("Unsupported arch: ${ARCH}")
 endif ()
 set(CMAKE_ASM_COMPILER      ${PREFIX}gcc)
@@ -43,6 +58,10 @@ set(CMAKE_ASM_FLAGS         ${CMAKE_C_FLAGS})
 set(CMAKE_C_LINK_FLAGS      "${LINK_FLAGS} -pthread")  # override default value to get rid of '-Wl,-search_paths_first -Wl,-headerpad_max_install_names'
 set(CMAKE_SHARED_LIBRARY_LINK_C_FLAGS)                  # override default value to get rid of '-rdynamic' on Linux
 
+if (KERNEL_HEADERS_DIR)
+	include_directories(${KERNEL_HEADERS_DIR}/include/uapi)
+endif ()
+
 # Execuatble
 foreach (PATH ${SRCS})
     get_filename_component(NAME ${PATH} NAME_WE)