build fixes for AdaCore libraries (#1747)

index/li/libgpr/libgpr-26.0.0.toml

@@ -16,7 +16,7 @@ GPR_BUILD = ["debug", "production", "coverage", "profiling"]
 LIBRARY_TYPE = ["static", "relocatable", "static-pic"]
 
 [[depends-on]]
-gnat = "/=2020" # Does not build with GNAT Community 2020
+gnat = ">=15" # Does not build with GNAT FSF 14
 xmlada = "~26.0.0"
 
 [origin]

index/li/libgpr2/libgpr2-26.0.0.toml

@@ -12,11 +12,15 @@ disabled = true
 
 [gpr-externals]
 GPR2_BUILD = ["debug", "release", "release_checks", "gnatcov"]
+GPR2_OS = ["Windows_NT", "UNIX"]
 LIBRARY_TYPE = ["static", "relocatable", "static-pic"]
 
 [gpr-set-externals]
 GPR2_BUILD = "release"
 
+[gpr-set-externals."case(os)".windows]
+GPR2_OS = "Windows_NT" # UNIX by default
+
 [[depends-on]]
 xmlada = "^26.0.0"
 gnatcoll = "^26.0.0"