Do not rely on the .git folder for TREXIO_DEVEL setup

This poses issues when packaging for Guix and Debian

CMakeLists.txt

@@ -16,7 +16,7 @@ set(CMAKE_C_STANDARD_REQUIRED ON)
 # Optional configure for developer mode to generate source code from org-mode files.
 option(TREXIO_DEVEL "TREXIO developer mode (for code generation)." OFF)
 
-if(EXISTS "${CMAKE_SOURCE_DIR}/.git/config")
+if(EXISTS "${CMAKE_SOURCE_DIR}/.devel")
   set(TREXIO_DEVEL ON)
   find_package(Python3 REQUIRED)
   if(Python3_FOUND)

configure.ac

@@ -16,11 +16,11 @@ AM_PROG_AR
 LT_PREREQ([2.2])
 LT_INIT
 
-# Activate developer mode when the source is the git repository.
+# Activate developer mode if a dummy file is present (true when cloning the git repository).
 # Otherwise, it is the source distribution and the developer mode should not be activated.
-TEST_IFEXISTS=".git"
-AC_CHECK_FILE([$TEST_IFEXISTS],
-        [enable_maintainer_mode="yes"],
+TEST_IFEXISTS=".devel"
+AS_IF([test -f $TEST_IFEXISTS],
+        [enable_maintainer_mode="yes"]
 )
 
 VERSION_MAJOR=`echo ${PACKAGE_VERSION} | cut -d. -f1`