Break CMake custom command dependency cycle

Ninja complains of dependency cycle due to separate custom commands
(which can end up building in parallel but the second has a dependency
on lemon parser build (the first).

| NOTE: VERBOSE=1 cmake --build oe/build/tmp-musl/work/armv5e-oe-linux-musleabi/jsonpath/git-r0/git --target all -- -j 8                 
| ninja: error: dependency cycle: contrib/lemon -> contrib/lemon     

We therefore update to a single add_custom_command that issues
the lemon build then uses it to generate the parser.c that we want.

Signed-of-by: Daniel F. Dickinson <cshored@thecshore.com>
Index: git/CMakeLists.txt
===================================================================
--- git.orig/CMakeLists.txt
+++ git/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 2.6)
+cmake_minimum_required(VERSION 3.0)
 
 PROJECT(jsonpath C)
 ADD_DEFINITIONS(-Os -Wall -Werror --std=gnu99 -Wmissing-declarations -Wno-error=unused-variable -ffunction-sections -D_GNU_SOURCE)
@@ -21,17 +21,11 @@ IF(JSONC_FOUND)
 ENDIF()
 
 ADD_CUSTOM_COMMAND(
-  OUTPUT contrib/lemon
-  DEPENDS contrib/lemon.c contrib/lempar.c
-  COMMAND gcc -o contrib/lemon contrib/lemon.c
-  COMMENT "Generating lemon parser generator"
-)
-
-ADD_CUSTOM_COMMAND(
   OUTPUT parser.c
-  DEPENDS parser.y contrib/lemon
+  COMMAND gcc -o contrib/lemon contrib/lemon.c
   COMMAND ./contrib/lemon parser.y
-  COMMENT "Generating parser.c"
+  DEPENDS contrib/lemon.c contrib/lempar.c parser.y
+  COMMENT "Generating lemon parser generator and using to generate parser.c"
 )
 
 FIND_PATH(ubox_include_dir libubox/list.h)