[meson] add dependency declaration for emper and emper_c

The variable names follow the naming convention for dependency variables.
Dependecy variables are needed for emper to be consumed as a meson subproject.
https://mesonbuild.com/Subprojects.html

emper/meson.build

@@ -45,6 +45,11 @@ emper = library(
   install: true,
 )
 
+emper_dep = declare_dependency(
+  include_directories : emper_all_include,
+  link_with : emper
+)
+
 
 emper_c_sources = [
   'c_emper.cpp',
@@ -57,3 +62,8 @@ emper_c = library(
   link_with: emper,
   install: true,
 )
+
+emper_c_dep = declare_dependency(
+  include_directories : emper_all_include,
+  link_with : emper_c
+)