Add explanation of LAPACK_STRLEN

Makefile.rule

@@ -134,6 +134,12 @@ VERSION = 0.3.27.dev
 # Build LAPACK Deprecated functions since LAPACK 3.6.0
 BUILD_LAPACK_DEPRECATED = 1
 
+# The variable type assumed for the length of character arguments when passing
+# data between Fortran LAPACK and C BLAS (defaults to "size_t", but older GCC
+# versions used "int"). Mismatches will not cause runtime failures but may result
+# in build warnings or errors when building with link-time optimization (LTO)
+# LAPACK_STRLEN=int
+
 # Build RecursiveLAPACK on top of LAPACK
 # BUILD_RELAPACK = 1
 # Have RecursiveLAPACK actually replace standard LAPACK routines instead of