diff --git unix/configure.ac unix/configure.ac
index 17da21848..d00edd3c1 100644
--- unix/configure.ac
+++ unix/configure.ac
@@ -185,6 +185,8 @@ if test x"${libtommath_ok}" = x -o x"${libtommath_ok}" != xno; then
     libtommath_ok=no])])
 fi
 AS_IF([test $libtommath_ok = yes], [
+  AC_SUBST(TCL_PC_REQUIRES_PRIVATE, ['libtommath >= 1.2.0,'])
+  AC_SUBST(TCL_PC_CFLAGS, ['-DTCL_WITH_EXTERNAL_TOMMATH'])
   AC_DEFINE(TCL_WITH_EXTERNAL_TOMMATH, 1, [Tcl with external libtommath])
 ], [
   AC_SUBST(TOMMATH_OBJS,[\${TOMMATH_OBJS}])
diff --git unix/configure unix/configure
index c0f7a32e7..ea8774fd5 100755
--- unix/configure
+++ unix/configure
@@ -737,6 +737,8 @@ RANLIB
 TOMMATH_INCLUDE
 TOMMATH_SRCS
 TOMMATH_OBJS
+TCL_PC_CFLAGS
+TCL_PC_REQUIRES_PRIVATE
 ZLIB_INCLUDE
 ZLIB_SRCS
 ZLIB_OBJS
@@ -5235,6 +5237,10 @@ fi
 if test $libtommath_ok = yes
 then :
 
+  TCL_PC_REQUIRES_PRIVATE='libtommath >= 1.2.0,'
+
+  TCL_PC_CFLAGS='-DTCL_WITH_EXTERNAL_TOMMATH'
+
 
 printf "%s\n" "#define TCL_WITH_EXTERNAL_TOMMATH 1" >>confdefs.h
 
diff --git unix/tcl.pc.in unix/tcl.pc.in
index 5f90c41ab..9c6da823e 100644
--- unix/tcl.pc.in
+++ unix/tcl.pc.in
@@ -10,7 +10,7 @@ Name: Tool Command Language
 Description: Tcl is a powerful, easy-to-learn dynamic programming language, suitable for a wide range of uses.
 URL: https://www.tcl-lang.org/
 Version: @TCL_VERSION@@TCL_PATCH_LEVEL@
-Requires.private: zlib >= 1.2.3, libtommath >= 1.2.0
+Requires.private: @TCL_PC_REQUIRES_PRIVATE@ zlib >= 1.2.3
 Libs: -L${libdir} @TCL_LIB_FLAG@ @TCL_STUB_LIB_FLAG@
 Libs.private: @TCL_LIBS@
-Cflags: -I${includedir}
+Cflags: -I${includedir} @TCL_PC_CFLAGS@
